Characterization of malT mutants that constitutively activate the maltose regulon of Escherichia coli.
The expression of the maltose regulon of Escherichia coli is controlled by a transcriptional activator, the product of the malT gene, and is induced by the presence of maltose or maltodextrins in the growth medium. We isolated eight mutants with mutations in malT which lead to constitutive expression of the regulon. The nucleotide sequences of the mutated genes revealed that the eight mutations are clustered in two small regions in the first one-third of the malT gene. Two mutated MalT proteins (corresponding to a mutation in each cluster) were purified and examined for in vitro activation of the MalT-dependent malPp promoter. Whereas wild-type MalT activity was absolutely dependent upon the presence of maltotriose, even at high protein concentrations, both mutated proteins were partially active in the absence of this sugar. Indeed, while the activity of the mutated proteins was still increased by maltotriose at low protein concentrations, the proteins were fully active in the absence of maltotriose at high protein concentrations. Both proteins exhibited a fivefold-higher affinity for maltotriose than the wild-type protein did.