Phage Mu Utilizes the Sliding Clamp for Late Gene Transcription

MuC, along with core RNAP and {sigma}70, is required for transcription of late genes of phage Mu. We observed that overexpression of MuC is lethal in E. coli and that host replication was over-initiating under these conditions. Suppressors of MuC lethality occurred in dnaA, diaA, and dnaX, genes involved in initiation of DNA replication. We hypothesized that lethality was likely due to increase in ATP-DnaA levels, which are normally kept in check by Hda-DnaN ({beta} clamp). We noticed that MuC harbors a well-defined motif for interaction with the {beta} clamp. We provide experimental support for this interaction and show that disrupting it through mutations in the clamp-binding motif on MuC, expressed from both plasmid and the phage genome, abrogated transcription from a MuC-dependent promoter as well as production of viable phage, respectively. Furthermore, we show that inactivation of {beta}-clamp specifically inactivates C-dependent transcription but not {sigma}70-dependent transcription. We conclude that MuC requires the b sliding clamp for transcribing late phage genes. To the best of our knowledge, this is the first demonstration of the involvement of the clamp, a processivity factor essential for DNA replication, for transcription by E. coli RNAP.