Double negative and positive control of tsx expression in Escherichia coli

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 This double negative control of Tsx synthesis was superceded by a positive control mechanism mediated by the cyclic AMP-catabolite activator protein (cAMP-CAP) complex.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.