VARIATION AND INVITRO SPLICING OF GROUP-I INTRONS IN RIBOSOMAL-RNA GENES OF PNEUMOCYSTIS-CARINII

The sequences of the rRNA genes of Pneumocystis carinii from rat and h uman sources demonstrate three distinct genotypes based on the group I introns present in these genes. One rat isolate (Pc1) contains such i ntrons in its 16S and 26S rRNA genes, while another rat isolate (Pc2) and a human isolate (Pc3) only contain an intron in the 26S rRNA gene. The four introns all catalyze their own excision from RNA transcripts , and this reaction is inhibited by the anti-pneumocystis drug pentami dine and its analogues. Although they differ in sequence, they are mor e similar to one another than to group I introns found in other eukary otic microbes.