The cyanobacterial origin and vertical transmission of the plastid tRNA^Leu group-I intron

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.