The dnaK/dnaJ operon of Haemophilus ducreyi contains a unique combination of regulatory elements

Haemophilus ducreyi, which causes the genital ulcer disease chancroid, requires high basal levels of the 60-kDa heat-shock (hs) protein GroEL in order to survive and adhere to host cells in the presence of common environmental stresses. In contrast, the 70-kDa hs protein, DnaK, a negative modulator of the hs response in prokaryotes, is not produced at as high a level as GroEL. Because of these differences, we were interested in identifying regulatory elements affecting the expression of the H. ducreyi dnaK/dnaJ operon. First, the genes encoding H. ducreyi DnaK (Hsp70) and DnaJ (Hsp40) were sequenced. The deduced amino acid sequences shared 82.8 and 63. 9% identity with the Escherichia coli DnaK and DnaJ homologs, respectively. Despite the presence of highly similar (but not identical) hs promoter sequences preceding both the H. ducreyi groES/groEL and dnaK/dnaJ operons, transcription levels for groEL were found to exceed that of dnaK. Subsequently, other genetic elements that could contribute to a lower basal expression of dnaK in H. ducreyi were identified. These elements include: (1) a complex promoter for dnaK consisting of four transcriptional start points (two for sigma32 and two for sigma70) identified by primer extension; (2) a putative binding site for Fur (a transcriptional repressor of iron-regulated genes) that overlaps the initiating AUG of dnaK; and (3) the potential for extensive secondary structure of the long leader sequences of the dnaK transcripts, which could interfere with efficient translation of DnaK. This unique combination of regulatory elements may be responsible for the relatively low-level expression of dnaK in this fastidious genital pathogen.