Dominant mutations of the cet gene of Escherichia coli result in tolerance to colicin E2 and increased amounts of an inner membrane protein with an Mr of 42,000. We have cloned the cet+ gene and sequenced its DNA, revealing that the gene product, coded by the longest open-reading frame, has an Mr of 49,772, with five predicted transmembrane structures towards its carboxy terminus and one at ist amino terminus. We have demonstrated that the cet locus does in fact code for the inner membrane protein that is present in increased amounts in cet mutants, and we have shown that this increased amount of Cet protein is the result of enhanced transcription. The cet gene is shown to be in the same operon as the phoM gene, which is required in a phoR background for expression of the structural gene for alkaline phosphatase, phoA. Although the Cet protein is not required for phoA expression, our experiments suggest that the Cet protein has an enhancing effect on the transcription of phoA. No effect of phosphate concentration on cet or phoM gene expression could be found and thus their primary function may not be connected to the phosphate regulon.
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