Transposable Mu-like phages in Firmicutes: new instances of divergence generating retroelements

Sequences of the most conserved proteins encoded by transposable (pro)phages were used to search recently sequenced Firmicute genomes for candidate transposable prophages. One complete Mu-like prophage, SglyMu-1, was identified, in four copies, in the Syntrophobotulus glycolicus DSM 8271 chromosome sequence. Related prophages were also found in partially assembled genomic sequences of other Firmicutes and newly sequenced Proteobacteria genomes, opening the road to the use of Mu-like derived genetic tools in Gram(+) bacteria. SglyMu-1 appears to carry a host variation system related to the DGR tropism switching retroelements, first characterized in Bordetella phages BPP-1, BIP-1 and BMP-1. Transposable phages are thus thriving among Firmicutes and can harness either of two host variation systems, the fiber genes inversion and reverse transcriptase-mediated site-directed, adenine-specific mutagenesis.