The accumulation of secretory protein precursors, caused either by mutations in secB or secA or by the overproduction of export-defective proteins, results in a two- to fivefold increase in the synthesis of heat shock proteins. In such strains, sigma 32, the alternative sigma factor responsible for transcription of the heat shock genes, is stabilized. The resultant increase in the level of sigma 32 leads to increased transcription of heat shock genes and increased synthesis of heat shock proteins. We have also found that although a secB null mutant does not grow on rich medium at a temperature range of 30 to 42 degrees C, it does grow at 44 degrees C. In addition, we found that a secB null mutant exhibits greater thermotolerance than the wild-type parental strain. Elevated levels of heat shock proteins, as well as some other non-heat shock proteins, may account for the partial heat resistance of a SecB-lacking strain.