Double negative and positive control of tsx expression in Escherichia coli

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Abstract
The Escherichia coli tsx gene encodes an outer membrane protein that is involved in nucleoside uptake and serves as the receptor protein for colicin K and several bacteriophages. Regulation of its expression was studied by using tsx-lacZ protein and operon fusion strains carrying mutations in deoR, cytR, and crp. The cytR-encoded repressor had a stronger influence on tsx transcription than the DeoR repressor did, and the level of tsx expression in a deoR cytR double mutant was approximately the sum of those found in the single deoR and cytR strains. This double negative control of Tsx synthesis was superceded by a positive control mechanism mediated by the cyclic AMP-catabolite activator protein (cAMP-CAP) complex. Our results suggest that tsx expression is controlled at two separate and differently regulated promoters: the weaker promoter (P1) is repressible by DeoR, while the stronger promoter (P2) is subject to negative and positive control by the CytR repressor and the cAMP-CAP complex, respectively. A mutant was isolated that showed unaltered tsx regulation by DeoR and the cAMP-CAP complex but strongly reduced repression by CytR. This tsx operator mutant was used to obtain a suppressor mutation located on a plasmid carrying the cloned cytR gene that restored CytR control of tsx expression. The direction of tsx transcription was determined and found to be counterclockwise on the E. coli chromosome.