Type III CRISPR-Cas systems can provide redundancy to counteract viral escape from type I systems.

CRISPR-Cas-mediated defense utilizes information stored as spacers in CRISPR arrays to defend against genetic invaders. We define the mode of target interference and role in antiviral defense for two CRISPR-Cas systems in Marinomonas mediterranea. One system (type I-F) targets DNA. A second system (type III-B) is broadly capable of acquiring spacers in either orientation from RNA and DNA, and exhibits transcription-dependent DNA interference. Examining resistance to phages isolated from Mediterranean seagrass meadows, we found that the type III-B machinery co-opts type I-F CRISPR-RNAs. Sequencing and infectivity assessments of related bacterial and phage strains suggests an 'arms race' in which phage escape from the type I-F system can be overcome through use of type I-F spacers by a horizontally-acquired type III-B system. We propose that the phage-host arms race can drive selection for horizontal uptake and maintenance of promiscuous type III interference modules that supplement existing host type I CRISPR-Cas systems.

Complete genome sequence of Marinomonas bacteriophage P12026.

Members of the genus Marinomonas in the Gammaproteobacteria are broadly distributed in marine environments where they could be infected by bacteriophages. Here we report the genome sequence of bacteriophage P12026 that can lytically infect bacterial strain IMCC12026, a member of the genus Marinomonas. To our knowledge, this is the first genome sequence of a lytic bacteriophage infecting the genus Marinomonas.