Molecular Biology and Evolution, Vol 15, 76-86, Copyright © 1998 by Society for Molecular Biology and Evolution
NA Doetsch, MD Thompson and RB Hallick
The fourth intron of the Euglena gracilis chloroplast photosystem II gene,
psbCi4, is a 1,605-bp twintron composed of two group III introns and a
coding locus for a 458-aa polypeptide, mat1, located in the internal
intron. psbCi4 homologs have been identified in seven euglenoids, including
E. myxocylindracea, E. viridis, E. deses, E. pisciformis, Cryptoglena
pigra, Eutreptia sp., and Lepocinclis beutschlii. All of the species
examined contain both the group III twintron and the mat1 locus, revealing
a more widespread occurrence of group III introns than previously known.
The L. beutschlii mat1 locus is interrupted by two novel mini-group II
introns of 224 and 258 nt, the smallest group II introns yet identified.
Reverse transcriptase polymerase chain reaction analysis confirmed the
splicing boundaries of the external and internal E. myxocylindracea, E.
viridis, and E. deses introns as well as the novel L. beutschlii mat1
introns. As determined by comparative phylogenetic analysis, group III
introns contain a structural homolog of group II intron domain VI. The mat1
loci encode peptide motifs characteristic of group II intron maturases. A
group III intron-encoded protein whose predicted sequence is similar to
group II intron-encoded maturases and a bona fide domain VI within group
III introns are compelling evidence for a common ancestor of group II and
group III introns.
ORIGINAL ARTICLE
A maturase-encoding group III twintron is conserved in deeply rooted euglenoid species: are group III introns the chicken or the egg?
Department of Molecular and Cellular Biology, University of Arizona, Tucson 85721, USA.
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