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

We have surveyed the distribution and reconstructed the phylogeny of the gr oup-I intron that is positioned in the anticodon loop of the tRNAL(Leu) gen e in cyanobacteria and several plastid genomes. Southern-blot and PCR analy ses showed that the tRNAL(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 tRNAL(Leu) group-I intron is absent from the red, cryptophyte and haptophyt e algae, although it is present in three members of the heterokont lineage. Phylogenetic analyses of the intron indicate that it was present in the cy anobacterial ancestor of the three primary plastid lineages, the Rhodophyta , Chlorophyta, and Glaucocystophyta. Its present-day distribution in plasti ds is consistent with a history of strictly vertical transmission, with no losses in land plants, several losses among green algae, and nearly pervasi ve loss in the Rhodophyta and its secondary derivatives.