Identification of five new essential genes involved in the synthesis of a secreted protein in Escherichia coli.

To define additional components of the export machinery of Escherichia coli, I have isolated extragenic suppressors of a mutant [secA(Ts)] that is temperature sensitive for growth and secretion at 37 degrees C. Suppressors that restored growth at 37 degrees C, but that rendered the cell cold sensitive for growth at 28 degrees C, were obtained. The suppressor mutations fall into at least seven loci, two of which (prlA and secC) have been previously implicated in protein secretion. The five remaining loci (ssaD, ssaE, ssaF, ssaG, and ssaH) have been mapped by P1 transduction and appear to define new genes in E. coli. All of the suppressor mutations allow both enhanced growth and protein secretion of the secA(Ts) mutant at 37 degrees C, but not 42 degrees C, indicating a continued requirement for SecA protein. Strains carrying solely the cold-sensitive mutations show reduced levels of certain periplasmic proteins when grown at low temperatures. In at least one case, that of maltose-binding protein, this defect is at the level of synthesis of the protein. Since mutants in any of seven genes as well as secA amber mutants halt or reduce the synthesis of an exported protein, it appears that E. coli may possess a general and complex mechanism for coupling protein synthesis and secretion.