Evidence for specificity at an early step in protein export in Escherichia coli

We previously described mutations in a gene, secB, which have pleiotropic effects on protein export in Escherichia coli. In this paper, we report the isolation of mutants in which the activity of the secB gene was eliminated. Null mutations in secB affected only a subset of exported proteins. Strains carrying these mutations, although unable to grow on L broth plates, were still viable on minimal media. These secB mutations reversed a block in the translation of an exported protein that was caused by the elimination of another component of the secretion machinery, SecA protein. These results suggest that the secB product acts at an early step in the export process and is involved in the export of only a subset of cell envelope proteins.