Suppression of growth and protein secretion defects in Escherichia coli secA mutants by decreasing protein synthesis.

We devised a new selection for conditionally lethal suppressors of secA mutants. This selection allows the isolation of both temperature-sensitive and cold-sensitive suppressor mutations, whereas previous studies were limited to nonlethal or cold-sensitive suppressor mutations. Two temperature-sensitive suppressor mutations lie in genes required for protein synthesis: asnS, the gene for the asparaginyl-tRNA synthetase, and divE, which encodes the tRNASer1. A previously characterized mutation in alaS, the gene for the alanyl-tRNA synthetase, suppresses the growth and secretion defects of a secA mutant. Although the primary effects of these suppressor mutations are different, it is likely that they cause suppression of secA mutations by altering the rate of protein synthesis, since the protein synthesis inhibitors, chloramphenicol and tetracycline, also suppress secA mutations. Chloramphenicol also suppresses the growth defect of certain other sec mutants. We postulate that the impaired secretory capacity of sec mutants can be offset by decreasing the rate of elongation of secreted proteins or by decreasing the total amount of secreted proteins per cell. The results indicate that our initial goal to identify cellular secretory components as suppressors of secA mutations might be difficult to achieve because of a high frequency of nonspecific suppressors that alter protein synthesis. Unexpectedly, the suppressor approach provides a direct genetic selection for mutants in protein synthesis.