THE CIRCE ELEMENT AND ITS PUTATIVE REPRESSOR CONTROL CELL-CYCLE EXPRESSION OF THE CAULOBACTER-CRESCENTUS-GROESL OPERON

The groESL operon is under complex regulation in Caulobacter crescentu s. In addition to strong induction after exposure to heat shock, under physiological growth conditions, its expression is subject to cell cy cle control. Transcription and translation of the groE genes occur pri marily in predivisional cells, with very low levels of expression in s talked cells. The regulatory region of groESL contains both a sigma(32 )-like promoter and a CIRCE element. Overexpression of C. crescentus s igma(32) gives rise to higher levels of GroEL and increased levels of the groESL transcript coming from the sigma(32)-like promoter. Site-di rected mutagenesis in CIRCE has indicated a negative role for this cis -acting element in the expression of groESL only at normal growth temp eratures, with a minor effect on heat shock induction. Furthermore, gr oESL-lacZ transcription fusions carrying mutations in CIRCE are no lon ger cell cycle regulated. Analysis of an hrcA null strain, carrying a disruption in the gene encoding the putative repressor that binds to t he CIRCE element, shows constitutive synthesis of GroEL throughout the Caulobacter cell cycle. These results indicate a negative role for th e hrcA gene product and the CIRCE element in the temporal control of t he groESL operon.