Mobility of the Sinorhizobium meliloti Group II Intron RmInt1 Occurs by Reverse Splicing into DNA, But Requires an Unknown Reverse Transcriptase Priming Mechanism

The mobile group II introns characterized to date encode ribonucleoprotein complexes that promote mobility by a major retrohoming mechanism in which the intron RNA reverse splices directly into the sense strand of a double-stranded DNA target site, while the intron-encoded reverse transcriptase/maturase cleaves the antisense strand and uses it as primer for reverse transcription of the inserted intron RNA. Here, we show that the Sinorhizobium meliloti group II intron RmInt1, which encodes a protein lacking a DNA endonuclease domain, similarly uses both the intron RNA and an intron-encoded protein with reverse transcriptase and maturase activities for mobility. However, while RmInt1 reverse splices into both single-stranded and double-stranded DNA target sites, it is unable to carry out site-specific antisense-strand cleavage due to the lack of a DNA endonuclease domain. Our results suggest that RmInt1 mobility involves reverse splicing into double-stranded or single-stranded DNA target sites, but due to the lack of DNA endonuclease function, it requires an alternate means of procuring a primer for target DNA-primed reverse transcription.