MBE Advance Access originally published online on October 18, 2007
Molecular Biology and Evolution 2008 25(1):92-100; doi:10.1093/molbev/msm232
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Research Articles |
Euglena Light-Harvesting Complexes Are Encoded by Multifarious Polyprotein mRNAs that Evolve in Concert
Department of Biology, University of New Brunswick, Fredericton, New Brunswick, Canada
E-mail: durnford{at}unb.ca.
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Abstract |
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Light-harvesting complexes (LHCs) are a superfamily of chlorophyll- and carotenoid-binding proteins that are responsible for the capture of light energy and its transfer to the photosynthetic reaction centers. Unlike those of most eukaryotes, the LHCs of Euglena gracilis are translated from large mRNAs, producing polyprotein precursors consisting of multiple concatenated LHC subunits that are separated by conserved decapeptide linkers. These precursors are posttranslationally targeted to the chloroplast and cleaved into individual proteins. We analyzed expressed sequence tags from Euglena to further characterize the structural features of the LHC polyprotein-coding genes and to examine the evolution of this multigene family. Of the 19 different LHC transcriptional units we detected, 17 encoded polyproteins composed of both tandem and nontandem repeats of LHC subunits; organizations that likely occurred through unequal crossing-over. Of the 2 nonpolyprotein-encoding LHC transcripts detected, 1 evolved from the truncation of a polyprotein-coding gene. Duplication of LHC polyprotein-coding genes was particularly important in the LHCI gene family where multiple paralogous sequences were detected. Intriguingly, several of the individual LHC-coding subunits both within and between transcriptional units appeared to be evolving in concert, suggesting that gene conversion has been a significant mechanism for LHC evolution in Euglena.
Key Words: light-harvesting complex • concerted evolution • polyprotein • Euglena
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Introduction |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionSupplementary MaterialAcknowledgementsReferences |
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The production of polyproteins is commonly associated with viruses where the viral genome is transcribed into a single mRNA molecule and translated into a large polyprotein. This polyprotein is then cleaved into individual proteins with distinct functions, often by a protease contained within the polyprotein itself (Carrington and Dougherty 1987
). Eukaryotic polyproteins, however, are uncommon, and they often possess a number of tandemly repeated protein subunits that have identical or related functions rather than proteins of unrelated functions as with viral polyproteins. The polyubiquitin gene, for instance, is commonly observed in eukaryotes, and this polyprotein contains a variable number of tandemly repeating, nearly identical ubiquitin units that are processed into individual proteins (Ozkaynak et al. 1984; Arribas et al. 1986; Graham et al. 1989). Profilaggrin, involved in the aggregation of keratin intermediate filaments during mammalian epidermal differentiation, is also expressed as a polyprotein precursor consisting of tandemly repeating units (Gan et al. 1990). Other examples include the lipid-binding proteins of nematodes that contain up to 10 repeating units (Kennedy 2000) and the antifreeze glycoproteins of Antarctic fish (Chen et al. 1997). In most cases, with the exception of polyubiquitin, the individual units of the polyprotein precursors are separated by polylinkers that act as processing sites (Rowan et al. 1996; Chen et al. 1997).
Euglena and many dinoflagellates are unique in that they contain chloroplasts that are surrounded by 3 membranes, though they obtained their plastids through independent secondary symbiotic events (Cavalier-Smith 1999). Additionally, both Euglena and dinoflagellates encode multiple nuclear-encoded plastid-targeted proteins translated as polyprotein precursors. These polyproteins are routed through the endomembrane system (Osafune et al. 1990) and directed to the chloroplasts via complex N-terminal targeting sequences (Durnford and Gray 2006). Once in the chloroplast, the individual proteins are liberated through proteolytic cleavage of conserved decapeptide linkers (polylinkers) (Muchhal and Schwartzbach 1992; Hiller et al. 1995).
In this study, we examined the complexity of the Euglena light-harvesting complex (LHC) gene family, which is known to produce LHCs as large polyproteins that are subsequently processed into individual units with the chloroplast (Houlné and Schantz 1987, 1988; Muchhal and Schwartzbach 1992). These polyproteins have previously been classified as LHCI or LHCII based upon whether their LHC subunits are predicted to associate exclusively with photosystem I (Houlné and Schantz 1988) or with photosystem II (Houlné and Schantz 1987; Muchhal and Schwartzbach 1992). Recently, we examined the antenna complexity and evolution in several Chl a/b-containing organisms and discovered a very diverse LHCII family in Euglena (Koziol et al. 2007). Here, we specifically examine the complexity of LHC polyproteins to determine LHC-subunit diversity, coding unit structure, and evolution of Euglena LHC units within individual polyprotein-encoding cDNAs.
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Materials and Methods |
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cDNA Libraries and Data Mining
cDNA libraries from Euglena gracilis (strain Z) were commercially prepared from RNA isolated under a variety of growth conditions. Bacterial plating, picking, DNA preparation, sequencing, trace processing, and data mining have been previously described in Koziol et al. (2007)
. Individual expressed sequence tags have been deposited in dbEST National Center for Biotechnology Information and the annotated clusters have been deposited in GenBank (table 1). The LHC sequence data were obtained from clustering 25,595 individual EST reads, and because many of the clusters contained related or nearly identical coding units, we confirmed the clustering in a number of ways. This included confirming the sizes of individual ESTs within a cluster by restriction mapping the largest cDNA and manually sequencing individual cDNAs in areas of limited sequence coverage.
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Phylogenetic Analyses
Phylogenetic analyses of LHC-nucleotide sequences were performed using Bayesian and maximum likelihood tests. MrModeltest 2.2 (Nylander 2004
) was used to select the best-fit model of nucleotide substitution for each of the analyses for the different alignments based upon the Akaike Information Criterion framework (Akaike 1974). The PHYML program (Guindon and Gascuel 2003; Guindon et al. 2005) was used for the maximum likelihood analyses (http://atgc.lirmm.fr/phyml/) utilizing the general time reversible (GTR) nucleotide substitution matrix, with gamma correction (6 categories), and accounting for the number of invariant sites.
MrBayes v3.1.2 (Huelsenbeck and Ronquist 2001) was used for the Bayesian analyses. GTR with 4 gamma distribution categories and incorporating the number of invariant sites was used as the substitution matrix. The data set was partitioned by codon to allow for different rates of change in each codon position. The number of generations performed was 5.00 x 106, with a sampling frequency of 100 and a 25% burn-in value. The consensus type was allcompat and posterior probabilities that support a node on the resulting consensus tree (>0.50) are shown. Bayesian inference was conducted using the resources of the Computational Biology Service Unit from Cornell University (http://cbsuapps.tc.cornell.edu/mrbayes.aspx). All phylogenetic trees are displayed using TreeView (Page 1996). Untrimmed alignments are available in the Supplementary Material online.
Nucleotide Substitution Analysis
The number of synonymous and nonsynonymous substitutions per site for several of the polyprotein-coding sequences was calculated using the K-Estimator program (Comeron 1999) with the Kimura 2-parameter model correcting for multiple hits and the confidence intervals (CIs) were calculated for the analyses. We also used GENECONV v1.81 (Sawyer 1999) to test for gene conversion events using statistical tests (Sawyer 1989). A mismatch penalty of 1 was used, and 10,000 random permutations of the polymorphic sites were executed in order to evaluate the significance of putative gene conversions. Sequence logo displays of the polylinkers were generated using the online program WebLogo (weblogo.Berkeley.edu/logo.cgi) (Crooks et al. 2004).
Northern Blot Analyses
To determine transcript size, individual cDNAs that constituted specific clusters were selected as probes for Northern hybridization such that the entire 3' untranslated regions (UTRs) plus a portion of the coding sequence preceding it were included. Single cDNA clones for select LHC-coding genes were isolated, and inserts were excised from the pCDNA3.1(+) vector by an EcoRI/XhoI double digest and purified from the gel (QIAquick gel extraction kit, Qiagen Mississauga Ontario, Canada). Probes were generated for Lhcbm8 (2.1 kb fragment from cDNA ELE00009515), Lhca1 (1.6 kb fragment from cDNA ELE00007187), and Lhcb4 (1.5 kb fragment from cDNA ELE00008952). Probes were labeled using dCTP 5'- [
-32P], hybridized to the blots, and detected as previously described (Durnford et al. 2003).
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Results |
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Polyprotein Organization and Evolution
We identified 3 polyprotein categories for both LHCII and LHCI: 1) complex proteins of divergent subunits, 2) tandem duplications of nearly identical subunits, and 3) tandem duplication of pairs of divergent subunits (figs. 1A and B). The previously described Euglena LHCI and LHCII polyproteins (Houlné and Schantz 1988
; Muchhal and Schwartzbach 1992) fall within the first category. We found evidence for extensive gene duplication for the category 1 LHCI polyproteins with 7 paralogs (Lhca1, 2, 4–8; fig. 1A). The full-length LHCI category 1 polyproteins contained 5 LHC-coding units per mRNA (fig. 1A), and the 3.9-kb transcript size was confirmed by Northern hybridization (fig. 2, Lhca1). Phylogenetic analysis of the LHCI category 1 polyproteins, however, indicated that the individual subunits encode divergent members of the LHC gene family with 4 of the 5 subunits falling into distinct subunit "types" (fig. 1C; 1, 2, 4, 3/5).
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We observed only a single LHCII category 1 polyprotein. This polyprotein was identical to the partial sequence found in GenBank (X61361 [GenBank] ; Muchhal and Schwartzbach 1992
), with the exception of a few mismatches that may be strain specific. The LHCII polyproteins are generally much larger than the LHCI polyproteins, and previous estimates have indicated that the transcript for the category 1 LHCII is 6.6 kb (Muchhal and Schwartzbach 1992). Taking into consideration the average size of the targeting sequence, the UTRs, and the size of the individual subunits, this would give a polyprotein with approximately 8 subunits (table 1). However, due to the large size of the transcripts, all the LHCII cDNAs in our libraries were truncated, and we were only able to detect a total of 4 subunits coded by Lhcbm1. For the single LHCII category 1 polyprotein, at least 3 distinct types were well supported (fig. 1D, I–III), a number that is likely to increase once the full-length sequence is available.
There was a single, divergent category 2 polyprotein-coding transcript with a weak association to the LHCI clade that had at least 3 tandemly repeated subunits (Lhca9) (fig. 1A and D). In comparison, we found numerous LHCII category 2 polyproteins, including Lhcbm3, 4, 6, 8–10 (fig. 1B). The subunits encoded by these transcripts also cluster within the main LHCII clade, excluding Lhcbm10 (fig. 1D). The estimates of the number of subunits per polyprotein are variable, ranging from 6 to 8 depending on the transcript (table 1). The transcript size for Lhcbm8 is 5.7 kb (fig. 2), which corresponds to approximately 7 subunits. Interestingly, the tandemly repeating subunits in transcripts Lhcbm4 and Lhcbm8 are nearly identical to 2 of the subunits in the category 1 polyprotein-coding gene LHCPII/Lhcbm1, and these form a supported clade on the LHCII tree (fig. 1D, type I). This close relationship is also apparent by weak hybridization to a 6.7-kb hybridization band detected with the Lhcbm8 probe (fig. 2), which corresponds to the main Lhcbm1 transcript size (Muchhal and Schwartzbach 1992). The 5.1-kb band detected with the Lhcbm8 probe (fig. 2) is likely from the Lhcbm4 transcript.
Lhca10 provides an example of a tandem arrangement of pairs of LHC subunits defining category 3 polyproteins. Lhca10 has at least 2 repeats of a pair of LHCI proteins (fig. 1A and D) and a transcript size of 4.2 kb (table 1), thus containing an estimated 6 subunits. The LHCII-related Lhcbm5 gene was also labeled a category 3 polyprotein by analogy to Lhca10, though we have insufficient sequence data to confirm the organization.
The unexpectedly high nucleotide sequence identity between the coding units of different polyprotein transcripts and between subunits within a polyprotein suggests that they are evolving in concert. For the LHCI category 1 sequences, it is clear that subunits 1, 2, and 4 from different paralogs form supported clades (fig. 1C). However, subunits 3 and 5 within each polyprotein gene are more similar to each other than they are to the paralogous subunits from different genes (fig. 1C, 3 + 5). We tested the hypothesis that these subunits were evolving in concert by calculating the synonymous (Ka) and nonsynonymous (Ks) changes per site between the sets of similar LHC subunits. Between the paralogous, yet divergent members of the LHCI-coding family, Lhca1 and Lhca5, there were significantly lower nonsynonymous and synonymous changes per site between subunits 3 and 5 within the polyproteins than compared with the paralogous subunits (3 vs. 3; 5 vs. 5) between the polyproteins (fig. 3, CI = 1%). Though the paralogous subunits between polyprotein genes continue to evolve independently of one another, subunits 3 and 5 within each paralogous gene appear to be evolving in concert. The remaining subunits (1, 2, and 4) had no significant intrapolyprotein subunit similarity and appeared to evolve independently of other subunits in the polyprotein. Moreover, homogenization events between subunits 3 and 5 within the LHCI polyproteins were predicted by GENECONV (P < 0.02) (Sawyer 1999). There is also evidence for concerted evolution of LHCII subunits, but in this case, the phenomenon extends to subunits of different transcripts in addition to those within the same transcript. Specifically, subunits 2 and 4 of Lhcbm1 (LHCPII) and the tandemly arranged subunits of Lhcbm4 and 8 together form a strongly supported type I clade in the LHCII tree (fig. 1D, I). All LHCII type I subunits had low Ka and Ks values (less than 0.00762 and 0.07128, respectively). Furthermore, using GENECONV, we found that there was strong evidence for these subunits evolving in concert (P < 0.05).
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The decapeptide linkers separating the coding units within the polyproteins all share similar features that can be divided into 3 sections: a front portion consisting of a variable (small, hydrophobic) residue; an invariant Ala, Met, and an aromatic amino acid (Phe or Tyr); a central portion consisting of 3 small and/or hydroxylated amino acids (Ala, Ser or Thr); and a terminal portion that consists of a conserved Gly, followed by a charged amino acid, and a basic residue (Lys or Arg) (fig. 4). There is high sequence conservation between linkers that precede identical or nearly identical subunits. For instance, all polylinkers in Lhcbm6 and 9 have a sequence of PAMFATAGRK and the polylinkers preceding subunits 3 and 5 in Lhca1 have a sequence of VAMFASSGHKD, suggesting that the homogenization events detected within the coding regions of these genes extend to the polylinkers (fig. 1A and B). This even applies to all the polylinkers preceding LHCII-type I subunits (fig. 1D) that represent both category 1 and 2 polyproteins having the sequence P/GAMFAASGRK, with the exception of subunit 2 of LHCPII where there is greater sequence variation (GAMLATSGRK).
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Evidence for the Creation of a LHC Gene Encoding a Single Subunit via Truncation/Deletion of an LHCI Polyprotein Gene
We discovered 2 cDNAs that encode only a single LHC subunit: Lhcb4 and Lhca3. Lhcb4 encodes the CP29 protein, a minor PSII-associated antenna, and the mRNA size was confirmed by Northern hybridization to be 1.4 kb (fig. 2). Lhca3 encodes a single LHCI subunit that closely resembles subunit 2 of the category 1 LHCI polyprotein-coding gene Lhca1 (figs. 1C and 5). Following the coding region of subunit 2 is a residual polylinker plus the first 25 amino acids of the N-terminal end of subunit 3, which is truncated by a premature stop codon. Interestingly, the similarity beyond the polylinker ends 8 amino acids before the stop codon, where there was a deletion leading to a frameshift and the introduction of a stop codon. The similarity between Lhca1 and Lhca3 was also obvious in the signal sequence/transit peptide (data not shown). There are 2 features that suggest the generation of Lhca3 was more complex than the introduction of a stop codon by a nonsense mutation, which would lead to the truncation a polyprotein-coding gene. The first is that though there is clear evidence for a conserved targeting domain and subunit 2, it appears as though subunit 1 has been excluded in the process of generating the transcript. The second feature is the limited similarity to the 3'UTR of Lhca1 in the sequence following the premature stop codon in Lhca3 (fig. 5). These findings together suggest that a series of deletions also accompanied the nonsense mutation in creating this LHCI transcript.
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Discussion |
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Polyprotein Organization and Origin
Eukaryotic polyproteins typically contain tandemly repeated coding units that yield mature proteins with highly conserved amino acid sequences. In Euglena, however, we observe 3 categories of polyprotein organization: 1) a complex arrangement of divergent subunits, 2) tandem duplications of nearly identical subunits, and 3) tandem duplications of pairs of divergent subunits, though the latter 2 categories are tentative until complete sequencing of the gene family is accomplished. The consistent tandem arrangement of the LHCs in large polyproteins suggests that unequal crossing-over was a driving force in the creation of the multisubunit-encoding genes. This is particularly obvious with the tandem repeats of nearly identical subunits or pairs of divergent subunits as found in categories 2 and 3, respectively. Unequal crossing-over during homologous recombination is a well-described mechanism for the generation of tandemly repeated DNA sequences in genomes (Metzenberg et al. 1991
; Propok'ev and Sukhodolets 2005). The creation of multisubunit polyproteins through the tandem duplication of individual subunits has been examined with the Euglena rbcS multigene family where, within a polyprotein-coding gene, multiple subunits have conserved intron insertion sites (Tessier et al. 1995), indicating that the subunits are duplicated segments that were incorporated into a single transcriptional unit. Though the partial LHCII genomic clone in GenBank lacked obvious conserved intron insertion sites, examination of the genomic organization of LHC gene family would have to be done to better assess how these large polyproteins were generated.
An obstacle to overcome during the arrangement of the LHC subunits as a polyprotein is the separation of the individual units prior to antenna assembly. In the polyproteins of Euglena, the individual coding units are delineated by polylinkers, of which there are 2 major categories: the decapeptide linkers observed in the LHCs and RuBisCO (Chan et al. 1990), and the tetrapeptide linkers with the consensus sequence "SVAM" described for the chloroplast phosphoglycerate kinase (PGK) polyproteins (Nowitzki et al. 2004). Although the presence of different recognition sequences would imply different proteases, the PGK polylinker is composed of amino acids that are chemically similar to the first portion of the LHC polylinkers as well as the 2 amino acids preceding the polylinkers (fig. 4: positions 1–4). In fact, an exact SVAM motif is found in the polylinkers of several LHCI proteins and is apparent in figure 4. Thus, despite containing shorter polylinkers, PGK may be processed by the same protease as the LHCs. N-terminal sequencing of Euglena LHCs would be useful in assessing exact processing sites.
The importance of gene duplications in the evolution of multigene families is well known (Hughes 1994). As with the LHC gene family in most eukaryotes, gene duplication is an important evolutionary mechanism in Euglena; in addition to the tandem duplications giving rise to the polyproteins, we also found evidence for the duplication of entire polyprotein-coding genes. The 7 category 1 LHCI polyproteins, for instance, are all paralogous as they possess a conserved order of LHC subunits (fig. 1A). Similar evidence for gene duplications was found for the LHCII category 2 polyproteins Lhcbm4/Lhcbm8 and Lhcbm6/Lhcbm9. Gene duplication creates functional redundancies, potentially allowing for the relaxed selection of one copy, that may ultimately lead to sub- or neofunctionalization of the redundant copy (Ohta 1987; Hurles 2004). Such subfunctionalization of LHC genes is well known for the plant and green algal antenna systems where different LHCs have specific interactions and functions within the photosystems (Jansson 1999). In fact, there is a comparable degree of divergence between the subunits within the category 1 polyproteins as found within the different plant/green algal LHCII proteins (ca. 50% amino acid identity). Thus, it is likely that the different subunits of the polyprotein precursors have equivalently complex functional roles in fine-tuning the light-harvesting capacity of the antenna systems.
Intriguingly, some subunits within polyproteins share higher sequence identity with each other than they do with subunits in paralogous polyproteins, indicating that there are homogenizing events acting on these subunits. Many multigene families (Archibald and Roger 2002; Pride and Blaser 2002; Bethke et al. 2006), including those that code for polyproteins (Sharp and Li 1987; Keeling and Doolittle 1995), are subject to sequence homogenization. This is possibly due to unequal crossing-over or biased gene conversion (Hillis et al. 1991), where gene conversion involves the nonreciprocal transfer of genetic information between highly similar sequences during homologous recombination (Abdulkarim and Hughes 1996). The repeated units of a gene family undergoing gene conversion events would evolve in concert with each other, accumulating fewer synonymous and nonsynonymous changes compared with paralogous genes in closely related organisms. The location of homologous genes within the genome appears to affect the level of homogenization as gene conversion occurs at increased frequency in genes closely arranged in head-to-head or tandem configurations (Benedict et al. 1996; Liao 1999). We speculate that the LHCI subunits 3 and 5, within the category 1 LHCI polyproteins are evolving in concert, as is LHCII subunit type I in Lhcbm1, Lhcbm4, and Lhcbm8. The subunits in the category 2 polyproteins Lhca9, Lhcbm6, Lhcbm9, and Lhcbm10 also appear to be evolving in concert at the intraprotein level. All homogenized subunits contain fewer synonymous changes between the subunits at the intrapolyprotein level than at the interprotein level (data not shown) and are predicted to have undergone gene conversion events (Sawyer 1999). The similarities between the type I subunit of Lhcbm4, Lhcbm8 (category 2 polyproteins), and Lhcbm1 (a category 1 LHCII polyprotein) are particularly interesting as it is possible that Lhcbm4 and Lhcbm8 are acting as donors for recombination, as is suggested for polyubiquitin (Catic and Ploegh 2005), which would allow the homogenization of subunit type I across all polyproteins possessing this subunit. There is also the possibility that the conservation is due to either purifying selection of an essential LHC subunit (Nei and Rooney 2005), as is proposed for the histone gene family (Piontkivska et al. 2002; Eirin-Lopez et al. 2004). However, the trend of highly conserved polylinkers preceding the conserved subunits would imply a homogenization as the polylinker sequence is expected to tolerate significant changes in primary sequence. In addition, LHCs are usually quite divergent in loop regions and a high level of selection in these regions would be unexpected. Examination of the chromosomal location and genomic sequence of these polyprotein-coding genes will help determine the exact extent of gene conversion or other mechanisms driving LHC evolution in Euglena.
Not All LHCs Are Encoded by Polyprotein Genes
It is interesting that there are 2 transcripts that do not encode polyproteins: Lhcb4 (CP29) (Koziol et al. 2007) and Lhca3. There is no evidence to suggest that Lhcb4 mRNA has ever encoded a polyprotein as it contains only a single-coding unit and lacks evidence of polylinkers. Lhca3, however, was generated through the rearrangement of a category 1 LHCI polyprotein-coding gene, through a series of internal deletions and the generation of a premature stop codon that resulted in a complete subunit connected to a truncated subunit by a polylinker (fig. 5). As this polylinker is conserved, the protein is likely proteolytically cleaved prior to its insertion into the membrane (Sulli and Schwartzbach 1996). It is possible that the Lhcb4 gene was generated in a similar manner, but through an ancient series of deletions, the evidence for which is no longer recognizable.
It is unknown why Euglena has polyprotein-coding genes for several gene families, though there are a few possible explanations. Initially, the arrangement of LHC-coding units into a polyprotein may have been to increase gene dosage in an organism with a reduced dependence of transcriptional regulatory mechanisms and a reliance on transsplicing to generate translatable mRNAs (McCarthy and Schwartzbach 1984; Keller et al. 1992), as proposed for the tandemly arranged genes in trypanosomes (Jackson 2007). Such an arrangement would allow for the production of a large number of protein subunits whose synthesis could be efficiently controlled by posttranscriptional mechanisms. This may have been linked to another rationale for polyprotein maintenance, one that is usually associated with viral production. The presence of variable numbers of distinct LHC types in polyproteins may assist in maintaining the proper stoichiometry of the different antenna proteins within the thylakoid membranes and to allow for the fine-tuning of their light-harvesting antennae. Though this is more likely to apply to the PSI antennae given that the category 1 polyproteins are predominant and encode for the distinct LHCI subunits that may make up the LHCI antenna belt. Nevertheless, any advantage of such an organization would have to be weighed against the disadvantages of polyprotein-coding genes. For instance, the introduction of frameshift or nonsense mutations in the upstream portion of a gene would deactivate multiple downstream-coding subunits, as we witnessed with Lhca3, and yielding pseudogenes (Catic and Ploegh 2005).
For both LHCI- and LHCII-coding genes, up to 17 distinct types were resolved by phylogenetic analyses and most encoded as polyprotein precursors. As it is generally accepted that Euglena acquired a plastid secondarily from a green alga and because no green alga described to date have antenna proteins organized into polyproteins, it is probable that the polyprotein conformation was created during the flood of green algal genes into the euglenoid nucleus during the endosymbiotic origin of the plastid. It is particularly interesting that dinoflagellates have independently evolved the use of polyproteins during the secondary origin of their plastids. Though the reason for this organization remains speculative, the large number of unique coding units generated in the process translate into a complex network of LHC subunits. This likely allowed for the fine-tuning of the light-harvesting apparatuses as different paralogs assumed specialized roles within the antenna systems and facilitated the origin of novel light-harvesting and photoprotective strategies.
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Supplementary Material |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionSupplementary MaterialAcknowledgementsReferences |
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Supplementary material is available at Molecular Biology and Evolution online (http://www.mbe.oxfordjournals.org).
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Acknowledgements |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionSupplementary MaterialAcknowledgementsReferences |
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We thank M.W. Gray for access to Euglena EST data, G.W. Saunders for helpful discussions, and D. Clark for reviewing an earlier version of this manuscript. This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada, Genome Atlantic, and Genome Canada.
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Footnotes |
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Geoffrey McFadden, Associate Editor
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References |
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