Abstract
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 Pmom has evolved to ensure irreversibility of the switch during the late lytic cycle of the phage.
Highlights
Regulation of transcription initiation is the major determining event employed by the cell to control gene expression and subsequent cellular processes
We have employed the promoter mutant and a pc mutant of the transactivator to address the overall mechanism of mom transcription activation
RNA polymerase (RNAP) binding to Pmom is not observed in the absence of the transactivator, and instead, a weak protection is observed at the P2 region (Fig. 1B, lane 2) [10]
Summary
Regulation of transcription initiation is the major determining event employed by the cell to control gene expression and subsequent cellular processes. Most activators stimulate transcription either by enhancing the binding of RNA polymerase (RNAP)3 to the promoter or open complex formation [1].
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