CIRCE, A NOVEL HEAT-SHOCK ELEMENT INVOLVED IN REGULATION OF HEAT-SHOCK OPERON DNAK OF BACILLUS-SUBTILIS

The dnaK and groESL operons of Bacillus subtilis are preceded by a pot ential sigma(43) promoter sequence (recognized by the vegetative a fac tor) and by an inverted repeat (IR) consisting of 9 bp separated by a 9-bp spacer. Since this IR has been found in many bacterial species, w e suspected that it might be involved in heat shock regulation. In ord er to test this hypothesis, three different mutational alterations of three bases were introduced within the IR preceding the dnaK operon. T hese mutations were crossed into the chromosome of B. subtilis, and ex pression of the dnaK and of the unlinked groESL operons was studied. T he dnaK operon exhibited increased expression at low temperature and a reduction in the stimulation after temperature upshift. Furthermore, these mutations reduced expression of the groESL operon at low tempera ture by 50% but did not interfere with stimulation after heat shock. T hese experiments show that the IR acts as a negative cia element of th e dnaK operon. This conclusion was strengthened by the observation tha t the IR reduced expression of two different transcriptional fusions s ignificantly after its insertion between the promoter and the reporter gene. Since this IR has been described in many bacterial species as p receding only genes of the dnaK and groESL operons, both encoding mole cular chaperones (39 eases are documented so far), we designated this heat shock element CIRCE (controlling IR of chaperone expression). Fur thermore, we suggest that this novel mechanism is more widespread amon g eubacteria than the regulation mechanism described for Escherichia c oli and has a more ancient origin.