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

Haemophilus ducreyi, which causes the genital ulcer disease chancroid, requ ires 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 environmenta l 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 Gr oEL. Because of these differences, we were interested in identifying regula tory elements affecting the expression of the H. ducreyi dnaK/dnaJ operon. First, the genes encoding H, ducreyi DnaK (Hsp70) and DnaJ (Hsp40) were seq uenced. The deduced amino acid sequences shared 82.8 and 63.9% identity wit h the Escherichia coli DnaK and DnaJ homologs, respectively. Despite the pr esence of highly similar (but not identical) hs promoter sequences precedin g both the H. ducreyi groES/groEL and dnaK/dnaJ operons, transcription leve ls 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. d ucreyi were identified. These elements include: (1) a complex promoter for dnaK consisting of four transcriptional start points (two for sigma(32) and two for sigma(70)) identified by primer extension; (2) a putative binding site for Fur (a transcriptional repressor of iron-regulated genes) that ove rlaps the initiating AUG of dnaK; and (3) the potential for extensive secon dary 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 exp ression of dnaK in this fastidious genital pathogen. (C) 1999 Published by Elsevier Science B.V. All rights reserved.