GROUP-I INTRONS WITH UNUSUAL SEQUENCES OCCUR AT 3 SITES IN NUCLEAR 18S RIBOSOMAL-RNA GENES OF ACANTHAMOEBA-LENTICULATA

Seven of eleven isolates of Acanthamoeba lenticulata were found to hav e group-I introns located at one of three positions within the 18S rRN A gene. The introns are 636-721-bp long and are absent from mature rRN A. They lack open reading frames that could encode any known endonucle ases. Sequences of introns from the same site in different isolates ar e 86.0-98.9% identical, while from different sites they are 24.2-29.8% identical. The most closely related introns from other organisms are in the 18S rRNA genes of several green algae where the 17.0-23.6% iden tity is mostly limited to a highly conserved core of base pairs includ ing P, Q, R and S. Because the ii. lenticulata introns only occur in o ne Acanthamoeba lineage, they were probably acquired after the diverge nce of this species.