NOTES A Group IIC-Type Intron Interrupts the rRNA Methylase Gene of

Group IIC introns insert next to the stem-loop structure of rho-independent transcription terminators, thus avoiding intact genes. The insertion sites of 17 copies of the G.st.I1 intron from Geobacillus stearothermophilus were compared. One copy of the intron was found to interrupt an open reading frame (ORF) encoding an rRNA methylase. Reverse transcriptase (RT)-encoding retroelements are an increasingly diverse collection of DNAs found in bacterial ge-nomes (4, 14). The most widely dispersed and most frequently encountered type of retroelement in bacterial genomes is the group II intron. Other types of bacterial retroelements include the multicopy single-stranded DNA (msDNA)-producing ret-rons (5) and the diversity-generating retroelements (9). Group II introns are self-splicing DNAs that code for a functional RT and also produce a functional RNA ribozyme. An mRNA containing a copy of the intron can fold into a catalytically active secondary structure and splice itself out o