Characterization of the heat shock response in Clostridium acetobutylicum has indicated that at least 15 proteins are induced by a temperature upshift from 30 to 42 degrees C. These so-called heat shock proteins include DnaK and GroEL, two highly conserved molecular chaperones. Several genes encoding heat shock proteins of C. acetobutylicum have been cloned and analysed. The dnaK operon includes the genes orfA (a heat shock gene with an unknown function), grpE, dnaK, and dnaJ; and the groE operon the genes groES and groEL. The hsp18 gene coding for a member of the small heat shock protein family constitutes a monocistronic operon. Interestingly, the heat shock response in this bacterium is regulated by a mechanism, which is obviously different from that found in Escherichia coli. So far, no evidence for a heat shock-specific sigma factor of the RNA polymerase in C. acetobutylicum has been found. In this bacterium, like in many Gram-positive and several Gram-negative bacteria, a conserved inverted repeat is located upstream of chaperone/chaperonin-encoding stress genes such as dnaK and groEL and may be implicated as a cis-acting regulatory site. The inverted repeat is not present in the promoter region of hsp18. Therefore, in C. acetobutylicum there are at least two classes of heat shock genes with respect to the type of regulation. Evidence has been found that a repressor is involved in the regulation of the heat shock response in C. acetobutylicum. However, this regulation seems to be independent of the inverted repeat motif, and the mechanism by which the inverted repeat motif mediates regulation remains to be elucidated.(ABSTRACT TRUNCATED AT 250 WORDS)