Dual role for transactivator protein C in activation of mom promoter of bacteriophage Mu

Transactivator C protein of bacteriophage Mu activates the mom gene of the phage by an unusual mechanism. DNA binding by C to its site results in unwinding of the neighboring sequences, realigning the out- of- phase promoter elements to facilitate RNA polymerase ( RNAP) binding. High level stimulation of a C- independent constitutive promoter mutant ( where RNAP is already bound) by the transactivator suggested an additional mechanism of transcription activation at a step after RNAP recruitment. In this study, we have investigated the various steps of promoter- polymerase interactions during transcription initiation by using both the promoter mutant and a positive control ( pc) mutant of C protein. The transactivator does not influence formation of the open complex or its stability after facilitating the RNAP binding. However, at a subsequent step, the protein exerts an important role, enhancing the promoter clearance by increasing the productive RNAP . promoter complex. The pc mutant of the transactivator C is compromised at this step, supporting the additional downstream role for C in mom transcription activation. We suggest that this unusual multistep activation of P-mom has evolved to ensure irreversibility of the switch during the late lytic cycle of the phage.