A MATURASE-ENCODING GROUP-III TWINTRON IS CONSERVED IN DEEPLY ROOTED EUGLENOID SPECIES - ARE GROUP-III INTRONS THE CHICKEN OR THE EGG

The fourth intron of the Euglena gracilis chloroplast photosystem II g ene, psbCi4, is a 1,605-bp twintron composed of two group III introns and a coding locus for a 458-aa polypeptide, matI, located in the inte rnal intron, psbCi4 homologs have been identified in seven euglenoids, including E. myxocylindracea, E. viridis, E. deses, E. pisciformis, C ryptoglena pigra, Eutreptia sp., and Lepocinclis beutschlii. All of th e species examined contain both the group III twintron and the mart lo cus, revealing a more widespread occurrence of group III introns than previously known. The L. beutschlii mat1 locus is interrupted by two n ovel mini-group II introns of 224 and 258 nt, the smallest group II in trons yet identified. Reverse transcriptase polymerase chain reaction analysis confirmed the splicing boundaries of the external and interna l E. myxocylindracea, E. viridis, and E. deses introns as well as the novel L. beutschlii mat1 introns. As determined by comparative phyloge netic analysis, group III introns contain a structural homolog of grou p II intron domain VI. The mat1 loci encode peptide motifs characteris tic of group II intron maturases. A group III intron-encoded protein w hose predicted sequence is similar to group II intron-encoded maturase s and a bona fide domain VI within group III introns are compelling ev idence for a common ancestor of group II and group III introns.