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J. Biol. Chem., Vol. 281, Issue 13, 8511-8517, March 31, 2006

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Dual Role for Transactivator Protein C in Activation of mom Promoter of Bacteriophage Mu^*

Atanu Chakraborty¹ and Valakunja Nagaraja²

From the Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore 560012, India and Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India

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.

Received for publication, December 2, 2005 , and in revised form, January 18, 2006.

^* This work was supported by a grant from the Department of Science and Technology, Government of India (to V. N.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ A Senior Research Fellow of the Council of Scientific and Industrial Research, India.

² To whom correspondence should be addressed: Dept. of Microbiology and Cell Biology, Indian Institute of Science, Bangalore 560012, India. Tel.: 91-80-2360-0668; Fax: 91-80-2360-2697; E-mail: vraj{at}mcbl.iisc.ernet.in.

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