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J. Bacteriol. doi:10.1128/JB.01618-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Mutations of the Act promoter in Myxococcus xanthus

Departments of Biochemistry and of Developmental Biology, Stanford University School of Medicine Stanford, CA 94305

^* To whom correspondence should be addressed. Email: kaiser{at}cmgm.stanford.edu.

	Abstract

Mutations within the -12 and -24 elements provide evidence that the act promoter is recognized by sigma-54 RNA polymerase. Deletion of the -20 base pair, which lies between the two conserved elements of sigma-54 promoters, decreased expression by 90%. In addition, mutation of a potential enhancer sequence, around -120, led to an 80% reduction in act gene expression. ActB, the second gene in the act operon, encodes a sigma-54 activator protein that is proposed to be an enhancer-binding-protein for the act operon. All act genes, actA to actE, are expressed together and constitute an operon, because an in-frame deletion of actB decreased expression of actA and actE to the same extent. After an initially slow phase of act operon expression, which depends on FruA, there is a rapid phase. The rapid phase is shown to be due to the activation of the operon expression by ActB, which completes a positive feedback loop. That loop appears to be nested within a larger positive loop in which ActB is activated by the C-signal via ActA, and the act operon activates transcription of the csgA gene. We propose that, as cells engage in more C-signaling, positive feedback raises the number of C-signal molecules per cell and drives the process of fruiting body development forward.

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