A Mutation Affecting the Regulation of a Seca-Lacz Fusion Defines a New Sec Gene

It was shown previously that the secA gene of Escherichia coli is derepressed in cells that have a defect in protein export. Here it is demonstrated that the beta-galactosidase produced by a secA-lacZ gene fusion strain is regulated in the same way. Studies on the fusion strain reveal that the promoter or a site involved in regulation of the secA gene is located considerably upstream from the structural gene. The properties of the fusion strain provide a new selection for mutants that are defective in protein export. Selection for increased lac expression of a secA-lacZ fusion strain yields mutations in three of the known sec genes, secA, secD and prlA/secY. In addition, mutations in several genes not previously known to affect secA expression were obtained. A mutation in one of these genes causes a pleiotropic defect in protein export and a cold-sensitive growth defect; this gene, which maps at approximately 90 min on the bacterial chromosome, has been named secE.