CLP is a homologue of cyclic AMP-receptor protein in Xanthomonas campestris. In this study, proteomic analysis and Western blotting showed that the clp mutant (TC820) of X. campestris synthesizes less GroESL proteins than the parental P20H. The groESL upstream regions, nt -583 to -32 (552 bp) and nt -178 to -29 (150 bp) relative to the groESL initiation codon, were cloned for transcriptional fusion assays. The 150-bp region, bearing putative sigma24- and sigma32-binding sites and the CIRCE element all known to regulate groESL operon, expressed the same levels of beta-galactosidase (300 U/ml) in both strains, indicating that CLP is not involved in the expression from this region. At early exponential phase, the 552-bp region displayed extremely high levels of promoter activity, 11,000 U/ml in P20H versus 5000 U/ml in TC820. The enzyme levels were about 2000 U/ml at stationary phase in both strains, indicating high levels of expression when cells cease growing. These results suggest that the sequence responding to CLP regulation resides between nt -178 and -583. However, since this region has no CLP-binding site and showed no binding to CLP in gel retardation assay, CLP is likely acting indirectly. This communication appears to be the first description of the positive regulation of a bacterial heat-shock operon by a CRP homologue.