Newly identified proviruses in Thermotogota suggest that viruses are the vehicles on the highways of interphylum gene sharing

Phylogenomic analyses of bacteria from the phylum Thermotogota have shown extensive lateral gene transfer with distantly related organisms, particularly with Firmicutes. One likely mechanism of such DNA transfer is viruses. However, to date, only three temperate viruses have been characterized in this phylum, all infecting bacteria from the Marinitoga genus. Here we report 17 proviruses integrated into genomes of bacteria belonging to eight Thermotogota genera and induce viral particle production from one of the proviruses. All except an incomplete provirus from Mesotoga fall into two groups based on sequence similarity, gene synteny and taxonomic classification. Proviruses of Group 1 are found in the genera Geotoga, Kosmotoga, Marinitoga, Thermosipho and Mesoaciditoga and are similar to the previously characterized Marinitoga viruses, while proviruses from Group 2 are distantly related to the Group 1 proviruses, have different genome organization and are found in Petrotoga and Defluviitoga. Genes carried by both groups are closely related to Firmicutes and Firmicutes (pro)viruses in phylogenetic analyses. Moreover, one of the groups show evidence of recent gene exchange and may be capable of infecting cells from both phyla. We hypothesize that viruses are responsible for a large portion of the observed gene flow between Firmicutes and Thermotogota.