Novel secA alleles improve export of maltose-binding protein synthesized with a defective signal peptide

Mutations previously designated prlD were described that suppressed malE signal sequence mutations and were located in the vicinity of the secA gene on the Escherichia coli chromosome. In this study, we demonstrated that four such independently isolated prlD mutations represented three unique single-base substitutions in secA, resulting in alterations at residues 111, 373, and 488 of the 901-residue SecA protein. Heretofore, the only mutations that had been described for secA were located early in the gene and resulted in a general protein export defect. Insertion mutations in the cloned gene X-secA operon that reduced or eliminated suppression by a prlD mutation also have been obtained. The properties of these suppressor and insertion mutations provide some insight into the role of SecA in the protein export process.