UC San Diego

We have discovered a prophage present in the unicellular model cyanobacterium Synechococcus elongatus PCC 7942 that represses biofilm formation and photosynthetic pigmentation under high light conditions. While previously identified as a small cryptic phage region, the 49 kb prophage containing over 51 open reading frames in fact encodes a complete phage genome including all genes necessary for generating a virion structure, switching between lysogeny and lysis, and replication and packaging of its genome. We have genetically deleted the prophage from the Synechococcus genome, generating a phage-less strain. Unlike the lysogenic strain, which remains planktonic in culture and bleaches under high light, the phage-less strain readily forms biofilms and does not bleach. We hypothesize that the non-lysogenic strain of Synechococcus naturally grows as a biofilm at the bottom of the water column, where lower light conditions would favor darker pigmentation for increased photon capture, and that the prophage actively releases cells from the biofilm into higher light conditions near the top of the water column in order to subsequently enable ecological dispersal of the phage.