MBE Advance Access published online on January 20, 2006
Molecular Biology and Evolution, doi:10.1093/molbev/msj097
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1 Department of Biology, Faculty of Sciences, Kyushu University, Fukuoka, Japan
* To whom correspondence should be addressed. The light-independent protochlorophyllide oxidoreductase (DPOR) plays a key role in the ability of non-flowering plants and algae to synthesize chlorophyll in darkness. This enzyme consists of three subunits encoded by the chlB, chlL, and chlN genes in the plastid genome. Previously, we found a high nonsynonymous substitution rate (dN) of the chlL gene in the lineage of Thuja standishii, a conifer belonging to the Cupressaceae. Here we revealed that the acceleration of dN in the chlL occurred as well in other species of Thuja, T. occidentalis and T. plicata. In addition, dark-grown seedlings of T. occidentalis were found to exhibit a pale yellowish color, and their chlorophyll concentration was much lower than that of other species of Cupressaceae. The results suggested that the species of Thuja have lost the ability to synthesize chlorophyll in darkness, and the functional constraint on the DPOR would thus be expected to be relaxed in this genus. Therefore, we expected to find that the evolutionary rates of all subunits of DPOR would in this case be accelerated. Sequence analyses of the chlN and chlB (encoding the other subunits of DPOR) in 18 species of Cupressaceae revealed that the dN of the chlN gene was accelerated in Thuja as was the dN of the chlL gene, but the dN of the chlB gene did not appear to differ significantly among the species of Cupressaceae. Sequencing of RT-PCR products of these genes showed that RNA editing was rare and unlikely to have contributed to the acceleration. Moreover, the RT-PCR analysis indicated that all chl genes were still transcriptionally active in T. occidentalis. Based on these results, it appears that species of Thuja still bear the DPOR protein, although the enzyme has lost its activity because of nonsynonymous mutations of some of the chl genes. The lack of acceleration of the dN of the chlB gene might be accounted for by various unknown functions of its gene product.
Accepted January 17, 2006
Research Article
Relaxation of Functional Constraint on Light-Independent Protochlorophyllide Oxidoreductase in Thuja
Junko Kusumi 1 *,
Aya Sato 2,
and
Hidenori Tachida 1
2 Research and Development Center for Higher Education, Kyushu University, Fukuoka, Japan
Junko Kusumi, E-mail: johtascb{at}mbox.nc.kyushu-u.ac.jp
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