A Cyanophage MarR-Type Transcription Factor Regulates Host RNase E Expression during Infection.

The marine picocyanobacterium contributes significantly to global primary production, and its abundance and diversity is shaped in part by viral infection. Here, we identified a cyanophage-encoded MarR-type transcription factor that induces the gene expression of host MED4 endoribonuclease (RNase) E during phage infection. The increase in transcript levels relies on the phage (p)MarR-mediated activation of an alternative promoter that gives rise to a truncated yet enzymatically fully functional RNase E isoform. In this study, we demonstrate that pMarR binds to an atypical activator site downstream of the transcriptional start site and that binding is enhanced in the presence of Ca ions. Furthermore, we show that dimeric pMarR interacts with the α subunit of RNA polymerase, and we identified amino acid residues S66, R67, and G106, which are important for Ca binding, DNA binding, and dimerization of pMarR, respectively.