EXTENSIVE STRUCTURAL CONSERVATION EXISTS AMONG SEVERAL HOMOLOGS OF 2 EUGLENA CHLOROPLAST GROUP-II INTRONS

82 of the 155 chloroplast introns in Euglena gracilis have been catego rized as group II introns. Because they are shorter and more divergent than group II introns from other organisms, the assignment of these E uglena introns to the group II class has been questioned. In the curre nt study, two homologs of E. gracilis petB intron 1 and four homologs of psbC intron 2 have been isolated from related species and character ized. Based on a comparative sequence analysis of intron homologs, the intron core and four of the six helical domains present in the canoni cal group II intron structural model are conserved in E. gracilis petB intron 1 and psbC intron 2 and all of their homologs. Distal portions of domain I, which are involved in most of the tertiary interactions, are less well conserved than the central core.