Expression of heat shock genes in Clostridium acetobutylicum

Abstract

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°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 cell 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 for 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. Another protein with a potential regulatory function might be the 21-kDa heat shock protein, which is induced significantly earlier than the majority of heat shock proteins. This protein has similarity to the redox carrier rubredoxin. Interestingly, heat shock genes are expressed in C. acetobutylicum at an increased rate not only after heat stress but also during the initiation of solvent formation. The mRNA level of some heat shock genes, e.g. dnaK, reached a maximum at the same time during the metabolic shift as the mRNA levels of genes necessary for solvent production. Therefore, the heat shock response in C. acetobutylicum might be part of a global regulatory network including different stress responses like heat shock, metabolic switch, and also sporulation.