Functional characterization of roles of GalR and GalS as regulators of the gal regulon.

An isorepressor of the gal regulon in Escherichia coli, GalS, has been purified to homogeneity. In vitro DNase I protection experiments indicated that among operators of the gal regulon, GalS binds most strongly to the external operator of the mgl operon, which encodes the high-affinity beta-methylgalactoside galactose transport system, and with less affinity to the operators controlling expression of the gal operon, which codes for enzymes of galactose metabolism. GalS has even less affinity for the external operator of galP, which codes for galactose permease, the major low-affinity galactose transporter in the cell. This order of affinities is the reverse of that of GalR, which binds most strongly to the operator of galP and most weakly to that of mgl. Our results also show that GalS, like its homolog, GalR, is a dimeric protein which in binding to the bipartite operators of the gal operon selectively represses its P1 promoter. Consistent with the fact that GalR is the exclusive regulator of the low-affinity galactose transporter, galactose permease, and that the major role of GalS is in regulating expression of the high-affinity galactose transporter encoded by the mgl operon, we found that the DNA binding of GalS is 15-fold more sensitive than that of GalR to galactose.