Genetic Characterization of a Novel Retron Element Isolated from Vibrio mimicus

Bacterial reverse transcriptase coding gene (RT) is essential for the production of a small satellite DNA-RNA complex called multicopy single-stranded DNA (msDNA). In this study, we found a novel retron, retron-Vmi1 (Vm85) from Vibrio mimicus. The retron is comprised of the msr-msd region, orf323, and the ret gene, a genetic organization similar to Salmonella's retron-Sen2 (St85). The protein sequence of the RNA-directed DNA polymerase (RT-Vmi1) is highly homologous to the RTs of Vibrio metoecus, Vibrio parahaemolyticus, and Vibrio vulnificus. Phylogenetic and protein sequence similarity analysis of retron-Vmi1 ORF323 and RT revealed a close relatedness to retron-Sen2. We found that retron-Vmi1 was inserted in the dusA gene, similar to the insertion of the retron-Vpa1 (Vp96) of V. parahaemolyticus AQ3354, suggesting that retrons can be transferred via the tRNA gene. These results are the first convincing evidence that retron is moving across species. The neighboring genes of retron-Vmi1 shared high homology with the genetic environment of V. parahaemolyticus and V. vulnificus retrons. We also found two junction points within the retron-Vmi1 and the dusA gene suggesting that retron-Vmi1 was inserted into this site in a two-step manner.