No abstract

We have surveyed the distribution and reconstructed the phylogeny of the group-I intron that is positioned in the anticodon loop of the tRNA(Leu) gene in cyanobacteria and several plastid genomes. Southern-blot and PCR analyses showed that the tRNA(Leu) intron is found in all 330 land plants that were examined. The intron was also found, and sequenced, in all but one of nine charophycean algae examined. Conversely, PCR analyses showed that the tRNA(Leu) group-I intron is absent from the red, cryptophyte and haptophyte algae, although it is present in three members of the heterokont lineage. Phylogenetic analyses of the intron indicate that it was present in the cyanobacterial ancestor of the three primary plastid lineages, the Rhodophyta, Chlorophyta, and Glaucocystophyta. Its present-day distribution in plastids is consistent with a history of strictly vertical transmission, with no losses in land plants, several losses among green algae, and nearly pervasive loss in the Rhodophyta and its secondary derivatives.

We sequenced the small-subunit (SSU) rDNA coding region and 1506 group I intron (that interrupts this gene) in Closterium ehrenbergii Menegh. ex Ralfs, Closterium littorale Gay, Cylindrocystis brebissonii (Menegh.) de Bary, Penium margaritaceum (Ehr.) de Bréb. ex Ralfs, and Staurastrum punctulatum de Bréb. and the 1506 intron in Sirogonium sticticum (J.E. Smith) Kütz. (Zygnematales). Reverse transcriptase (RT)/PCR analyses demonstrated that the 1506 introns are not present in vivo in mature rRNA. Splicing analyses in vitro showed, however, that these introns do not catalyze their own excision from the rRNA coding region. Mutation of the conserved G (G) to an A at the 3 terminus of the 1506 intron of Gonatozygon aculeatum Hastings (Zygnematales) restored self-splicing activity. This mutant intron utilized a new 3 splice site to facilitate autoexcision. We speculate that the 3 terminus of the wild-type 1506 intron may be a target for a ''helper'' factor that facilitates the second step of splicing (exon ligation) in vivo in the Zygnematales. Phylogenetic analyses showed congruence of rDNA and intron trees confirming an ancient origin of the 1506 intron in the common ancestor of the Zygnematales. The rDNA trees were compared to those inferred from rbcL sequence analyses. These trees were in general agreement and showed polyphyly of the Mesotaeniaceae and the Zygnemataceae. The 1506 introns contain significant evolutionary signal and provided strong phylogenetic support for groups within the Zygnematales when analyzed alone or in combination with the SSU rDNA coding regions. The secondary structure of 1506 group I introns was studied to identify RNA elements that may be useful as systematic markers. This analysis showed an optional helix found in RNA domain P2 that was lost in a monophyletic group of Desmidiaceae. This helix is found in all other 1506 group I introns, including those interrupting red algal and fungal SSU rDNAs.