Modulating Action of Cyclic-amp on the Expression of 2 Sos Genes in Escherichia-coli K-12

The expression of the linkedtoxA andtoxB genes coding for the A and B subunits of the heat-labile enterotoxin ofEscherichia coli, respectively, has been studied. For this reason,toxA-lacZ andtoxB-lacZ fusions were constructed through a promoter-probe vector. Results obtained show that thetoxB gene has a promoter from which it may be transcribed independently from thetoxA gene. Nevertheless, the expression oftoxB gene is about 25-fold higher from thetoxA promoter than from its own promoter. Furthermore, the presence of glucose in the culture medium decreased the transcription from bothtoxA andtoxB promoters. Likewise,cya andcrp mutants exhibited a lower level oftox gene expression than did the wild-type strain. The addition of cyclic AMP increased the expression fromtoxA andtoxB promoters in bothcya and wild-type strains, but not in acrp mutant. All these data suggest that the cyclic AMP receptor protein-cyclic AMP complex positively modulatestoxA andtoxB gene transcription inE. coli.