S-16. Irreversible genetic switch to transactivate mom gene during lytic cycle of bacteriophage Mu

Expression of genes is regulated by ‘on’ and ‘off’ mechanisms executed at various levels. At the transcriptional level, the positive regulators (activators) turn ‘on’ gene expression while negative regulators (repressors) turn ‘off’ the expression. The genetic switches essentially achieve either induction or repression driven by the cellular requirement mediated by timely signaling events. The reversibility of the well characterized control circuits ensure optimized expression of genes. I will describe a genetic switch which appears to be irreversible.

Transactivator C protein of bacteriophage Mu activates mom gene of the phage by a distinct multi-step mechanism. Site specific DNA binding by C results in untwisting of the neighboring promoter sequences, realigning the out of phase elements to facilitate RNA polymerase (RNAP) binding. By employing the promoter mutant and a positive control (pc) mutant of C protein, we demonstrate that the transactivator does not influence formation of open complex or its stability after facilitating the RNAP binding. The protein enhances the promoter escape 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. Interaction between RNAP and C occurring on DNA enhances promoter escape achieving a strong and irreversible activation of mom transcription. This unusual multi-step activation of the ‘late promoter’ would ensure irreversibility of the switch during late lytic cycle of the phage.