ExpA: a conditional mutation affecting the expression of a group of exported proteins in Escherichia coli K-12

A mutant of Escherichia coli K-12 was isolated as conditionally deficient in the expression of two exported proteins simultaneously (i.e. two acid phosphatases). The mutant was found to be thermosensitive on minimal medium at 37 degrees C and above, but grew normally on rich media at these temperatures. The mutation, named expA and located at 22 min on the recalibrated linkage map, depressed the levels of six periplasmic enzymatic activities in bacteria grown at 37 degrees C. At least ten proteins were greatly reduced in the periplasm under these conditions. The mutation also affected some outer membrane proteins, among which were the ompF protein and a protein which may be protein III, but had little effect on cytoplasmic membrane proteins. The gel patterns of the soluble cytoplasmic proteins were not modified except for one major protein of MW 47,000. The activities of beta-galactosidase and of aspartate transcarbamylase were unmodified. After growth at 30 degrees C no difference was observed between expA and expA+ isogenic strains. The results are discussed with respect to the mechanism of protein export.