Heat-shock proteins can substitute for SecB function during protein export in Escherichia coli

In this study we have shown that (i) induction of the heat-shock response can substitute for SecB function in Escherichia coli, (ii) SecB itself is not a heat-shock protein and (iii) a basal level of heat-shock proteins is required for cells to grow in the absence of a functional SecB protein. Overproduction of DnaK, or GroEL/ES, which were candidates for the heat-shock proteins that could substitute for SecB function, did not rescue the export defect caused when SecB was limiting or absent. In an attempt to identify the heat-shock protein(s) which could substitute for SecB function, unlinked suppressors of secB were isolated and characterized. Interestingly, most of the suppressors mapped to the rpoH locus. Since rpoH encodes sigma 32, the heat-shock transcription factor, it is likely that these suppressors affect the synthesis levels of heat-shock proteins that can substitute for SecB function. The remaining suppressors did not map to any known heat-shock or export genes. Collectively, our data suggest that these suppressors may represent unidentified heat-shock proteins or export factors that act in a manner similar to SecB in facilitating the export process in E. coli.