EXPRESSION OF HEAT-SHOCK GENES IN CLOSTRIDIUM-ACETOBUTYLICUM

Characterization of the heat shock response in Clostridium acetobutyli cum has indicated that at least 15 proteins are induced by a temperatu re upshift from 30 to 42 degrees C. These so-called heat shock protein s include DnaK and GroEL, two highly conserved molecular chaperones. S everal genes encoding heat shock proteins of C. acetobutylicum have be en cloned and analysed. The dnaK operon includes the genes orfA (a hea t 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 mem ber of the small heat shock protein family constitutes a monocistronic operon. Interestingly, the heat shock response in this bacterium is r egulated by a mechanism, which is obviously different from that found in Escherichia coli. So far, no evidence for a heat shock-specific sig ma factor of the RNA polymerase in C. acetobutylicum has been found. I n this bacterium, like in many Gram-positive and several Gram-negative bacteria, a conserved inverted repeat is located upstream of chaperon e/chaperonin-encoding stress genes such as dnaK and groEL and may be i mplicated as a cis-acting regulatory site. The inverted repeat is not present in the promoter region of hsp18 Therefore, in C. acetobutylicu m there are at least two classes of heat shock genes with respect to t he type of regulation. Evidence has been found that a repressor is inv olved in the regulation of the heat shock response in C, acetobutylicu m. However, this regulation seems to be independent of the inverted re peat motif, and the mechanism by which the inverted repeat motif media tes regulation remains to be elucidated. Another protein with a potent ial regulatory function might be the 21-kDa heat shock protein, which is induced significantly earlier than the majority of heat shock prote ins. This protein has similarity to the redox carrier rubredoxin. Inte restingly, heat shock genes are expressed in C. acetobutylicum at an i ncreased rate not only after heat stress but also during the initiatio n 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 globa l regulatory network including different stress responses like heat sh ock, metabolic switch, and also sporulation.