Summary: The maltose regulon consists of five operons under the control of the MalT transcriptional activator. operon fusions were constructed with the MalT-dependent promoter and with the promoter itself. β-Galactosidase activity displayed by these fusions during growth at different external pH (pH) revealed that growth at a pH higher than 6 stimulates the transcription of - and MalT-controlled genes in the absence or presence of maltose. Using a promoter that is cAMP-CRP (cAMP receptor protein)-independent, it was demonstrated that CRP is essential for pH regulation and that the pH-dependent activity of is a direct consequence of regulation. The pH regulation displayed by a deleted but still functional promoter fused to demonstrates that this minimal promoter contains all the regulatory regions for establishing pH regulation. In the absence of Mlc, a repressor of expression, the pH regulation of was still effective. It is proposed that binding of cAMP-CRP at may be affected by topology induced by pH or that a pH-dependent effector may act in concert with the cAMP-CRP complex.


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