Conserved small RNAs govern replication and incompatibility of a diverse new plasmid family from marine bacteria

Plasmids are autonomously replicating extrachromosomal DNA molecules that often impart key phenotypes to their bacterial hosts. Plasmids are abundant in marine bacteria, but there is scant knowledge of the mechanisms that control their replication in these hosts. Here, we identified and characterized the factors governing replication of a new family of plasmids from marine bacteria, typified by the virulence-linked plasmid pB1067 of Vibrio nigripulchritudo. Members of this family are prevalent among, yet restricted to, the Vibrionaceae. Unlike almost all plasmid families characterized to date, the ori regions of these plasmids do not encode a Rep protein to initiate DNA replication; instead, the ori regions encode two partially complementary RNAs. The smaller, termed RNA I, is ∼68-nt long and functions as a negative regulator and the key determinant of plasmid incompatibility. This Marine RNA-based (MRB) plasmid family is the first characterized family of replicons derived from marine bacteria. Only one other plasmid family (the ColE1 family) has previously been reported to rely on RNA-mediated replication initiation. However, since the sequences and structures of MRB RNA I transcripts are not related to those of ColE1 replicons, these two families of RNA-dependent replicons likely arose by convergent evolution.