Transcription of a ‘photosynthetic’ T4‐type phage during infection of a marine cyanobacterium

The transcription of S-PM2 phage following infection of Synechococcus sp. WH7803, a marine cyanobacterium, was analysed by quantitative real-time PCR. Unlike the distantly related coliphage T4, there were only two (early and late) instead of three (early, middle and late) classes of transcripts during the developmental cycle of the phage. This difference is consistent with the absence from the S-PM2 genome of T4-like middle mode promoter sequences and the transcription factors associated with their recognition. Phage S-PM2 carries the 'photosynthetic' genes psbA and psbD that encode homologues of the host photosystem II proteins D1 and D2. Transcripts of the phage psbA gene appeared soon after infection and remained at high levels until lysis. Throughout the course of infection, the photosynthetic capacity of the cells remained constant. A considerable transient increase in the abundance of the host psbA transcripts occurred shortly after infection, suggesting that the host responds to the trauma of phage infection in a similar way as it does to a variety of other environmental stresses. The very substantial transcription of the phage psbA gene during the latter phase of phage infection suggests that S-PM2 has acquired this cellular gene to ensure that D1 levels and thus photosynthesis are fully maintained until the infected cell finally lyses. Unexpectedly, transcripts of a phage-encoded S-layer protein gene were among the earliest and most abundant detected, suggesting that this partial homologue of a host protein plays an important role in the S-PM2 infection process.