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J. Bacteriol. doi:10.1128/JB.00315-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Switching control of the expression of ptsG from the Mlc regulon to the NagC regulon

Institut de Biologie Physico-Chimique, UPR9073-CNRS, 13 rue Pierre et Marie Curie, 75005 Paris

^* To whom correspondence should be addressed. Email: Jackie.Plumbridge{at}ibpc.fr.

	Abstract

The Mlc and NagC transcriptional repressors bind to similar 23 bp operators. The sequences are weakly palindromic with just 4 positions totally conserved. There is no cross regulation observed between the repressors in vivo but there are no obvious bases which could be responsable for operator site discrimination. To investigate the basis for operator recognition and to try to understand what differentiates NagC sites from Mlc sites we have undertaken mutagenesis experiments to convert the ptsG gene from being regulated by Mlc into a gene regulated by NagC. There are two Mlc operators upstream of ptsG and to switch ptsG to the NagC regulon, it was necessary to change two different characteristics of both operators. Firstly, we replaced the A-T bp, at position +/-11 from the center of symmetry of the operators, for G-C bp. Secondly, we changed the sequence of CG bp in the central region of the operator (positions -4 to +4 around the center of symmetry). Our results show that changes at either of these locations are sufficient to lose regulation by Mlc but that both types of changes in both operators are necessary to convert ptsG to a gene regulated by NagC. In addition these experiments confirmed that two operators are necessary for regulation by NagC. We also show that regulation of ptsG by Mlc involves some co-operative binding of Mlc to the two operators.