Phylogeny and self-splicing ability of the plastid tRNA-Leu group I Intron.
Group I introns are mobile RNA enzymes (ribozymes) that encode conserved primary and secondary structures required for autocatalysis. The group I intron that interrupts the tRNA-Leu gene in cyanobacteria and plastids is remarkable because it is the oldest known intervening sequence and may have been present in the common ancestor of the cyanobacteria (i.e., 2.7-3.5 billion years old). This intron entered the eukaryotic domain through primary plastid endosymbiosis. We reconstructed the phylogeny of the tRNA-Leu intron and tested the in vitro self-splicing ability of a diverse collection of these ribozymes to address the relationship between intron stability and autocatalysis. Our results suggest that the present-day intron distribution in plastids is best explained by strict vertical transmission, with no intron losses in land plants or a subset of the Stramenopiles (xanthophyceae/phaeophyceae) and frequent loss among green algae, as well as in the red algae and their secondary plastid derivatives (except the xanthophyceae/phaeophyceae lineage). Interestingly, all tested land plant introns could not self-splice in vitro and presumably have become dependent on a host factor to facilitate in vivo excision. The host dependence likely evolved once in the common ancestor of land plants. In all other plastid lineages, these ribozymes could either self-splice or complete only the first step of autocatalysis.
Simon
Dawn
D
Department of Biological Sciences and Center for Comparative Genomics, University of Iowa, 210 Biology Building, Iowa City, IA 52242-1324, USA.
Fewer
David
D
Friedl
Thomas
T
Bhattacharya
Debashish
D
D056890:Eukaryota / Q000235:genetics; D007438:Introns / Q000235:genetics*; D008969:Molecular Sequence Data; D010802:Phylogeny; D010944:Plants / Q000235:genetics; D018087:Plastids / Q000235:genetics*; D012322:RNA Precursors / Q000235:genetics; D012326:RNA Splicing / Q000235:genetics*; D012356:RNA, Transfer, Leu / Q000235:genetics*
D012322:RNA Precursors; D012356:RNA, Transfer, Leu