Bacteria have numerous distinctive groups of phage-plasmids with conserved phage and variable plasmid gene repertoires

Plasmids and temperate phages are key contributors to bacterial evolution. They are usually regarded as very distinct. However, some elements, termed phage-plasmids, are known to be both plasmids and phages, e.g. P1, N15 or SSU5. The number, distribution, relatedness and characteristics of these phage-plasmids are poorly known. Here, we screened for these elements among ca. 2500 phages and 12000 plasmids and identified 780 phage-plasmids across very diverse bacterial phyla. We grouped 92% of them by similarity of gene repertoires to eight defined groups and 18 other broader communities of elements. The existence of these large groups suggests that phage-plasmids are ancient. Their gene repertoires are large, the average element is larger than an average phage or plasmid, and they include slightly more homologs to phages than to plasmids. We analyzed the pangenomes and the genetic organization of each group of phage-plasmids and found the key phage genes to be conserved and co-localized within distinct groups, whereas genes with homologs in plasmids are much more variable and include most accessory genes. Phage-plasmids are a sizeable fraction of the sequenced plasmids (similar to 7%) and phages (similar to 5%), and could have key roles in bridging the genetic divide between phages and other mobile genetic elements.

Automated summary

Plasmids and temperate phages play crucial roles in bacterial evolution. Some elements, known as phage-plasmids, have been identified as both plasmids and phages. A study identified 780 phage-plasmids among a large number of phages and plasmids, grouped them into different categories, and suggested their ancient origins.