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Journal of Bacteriology, February 2009, p. 701-712, Vol. 191, No. 3
0021-9193/09/$08.00+0     doi:10.1128/JB.00767-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Glucose- and Glucokinase-Controlled mal Gene Expression in Escherichia coli{triangledown}

Christina Lengsfeld, Stefan Schönert, Renate Dippel, and Winfried Boos*

Department of Biology, University of Konstanz, 78457 Konstanz, Germany

Received 30 May 2008/ Accepted 10 November 2008

MalT is the central transcriptional activator of all mal genes in Escherichia coli. Its activity is controlled by the inducer maltotriose. It can be inhibited by the interaction with certain proteins, and its expression can be controlled. We report here a novel aspect of mal gene regulation: the effect of cytoplasmic glucose and glucokinase (Glk) on the activity and the expression of MalT. Amylomaltase (MalQ) is essential for the metabolism of maltose. It forms maltodextrins and glucose from maltose or maltodextrins. We found that glucose above a concentration of 0.1 mM blocked the activity of the enzyme. malQ mutants when grown in the absence of maltodextrins are endogenously induced by maltotriose that is derived from the degradation of glycogen. Therefore, the fact that glk malQ+ mutants showed elevated mal gene expression finds its explanation in the reduced ability to remove glucose from MalQ-catalyzed maltodextrin formation and is caused by a metabolically induced MalQ– phenotype. However, even in mutants lacking glycogen, Glk controls endogenous induction. We found that overexpressed Glk due to its structural similarity with Mlc, the repressor of malT, binds to the glucose transporter (PtsG), releasing Mlc and thus increasing malT repression. In addition, even in mutants lacking Mlc (and glycogen), the overexpression of glk leads to a reduction in mal gene expression. We interpret this repression by a direct interaction of Glk with MalT concomitant with MalT inhibition. This repression was dependent on the presence of either maltodextrin phosphorylase or amylomaltase and led to the inactivation of MalT.

* Corresponding author. Mailing address: Department of Biology, University of Konstanz, 78457 Konstanz, Germany. Phone: 0049-7531-882658. Fax: 0049-7531-883356. E-mail: winfried.boos{at}uni-konstanz.de

{triangledown} Published ahead of print on 21 November 2008.

Journal of Bacteriology, February 2009, p. 701-712, Vol. 191, No. 3
0021-9193/09/$08.00+0     doi:10.1128/JB.00767-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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