Specificities and commonalities of the Planctomycetes plasmidome

Although plasmids play a crucial role in antibiotic resistance and contemporary biotechnology, our comprehension of their natural ecological dynamics remains restricted to a few bacterial groups. Planctomycetes is a bacterial phylum with unusual molecular and cellular biology for which little is known about its plasmidome. This study provides the first comprehensive description of the diversity of the endogenous plasmids found in Planctomycetes, which may be used as starting points to generate different genetic tools for research on this phylum. Plasmids from Planctomycetes encode a wide variety of biological functions, although for a large portion of the encoded genes a function cannot be assigned, which is usual in Planctomycetes. They seemed to have largely coevolved with the genome of their hosts, with which they share many homologs. We also detected recent transfer events of insertion sequences between co-habiting chromosomes and plasmids. 60% of the plasmid genes are distantly related to chromosomally-encoded genes and 40% have homologs in plasmids from other bacterial groups, while 36% of the proteins composing the planctomycetal plasmidome are exclusive. Most planctomycetal plasmids encode a replication initiation protein of the RPA family in the proximity of a putative iteron-containing replication origin, as well as active type I partition systems. One conjugative and three mobilizable plasmids were identified, suggesting horizontal gene transfer via conjugation in this phylum. ### Competing Interest Statement The authors have declared no competing interest.