We cloned the DNA region of the Vibrio harveyi chromosome containing the heat shock genes dnaK and dnaJ and sequenced them. These genes are arranged in the chromosome in the order dnaK-dnaJ, as in other proteobacteria of the alpha and gamma subdivisions. The dnaK gene is 1923 nucleotides in length and codes for a protein of 640 amino acid residues, with a predicted molecular mass of 69,076 Da and 81.2% similarity to the DnaK protein of Escherichia coli. The V. harveyi dnaJ gene has a coding sequence of 1158 nucleotides. The predicted DnaJ protein contains 385 amino acids, its calculated molecular mass is 41,619 Da and it has 74.7% similarity to the DnaJ protein of E. coli. Northern hybridization experiments with RNA from V. harveyi cells and a DNA probe carrying both the dnaK and dnaJ genes showed a single, heat-inducible transcript, indicating that these genes form an operon. Primer extension analysis revealed five heat-inducible transcriptional start sites upstream of the dnaK gene, two of which (T1 and T4) are preceded by sequences typical of the E. coli heat shock promoters recognized by the sigma 32 (sigma32) factor. Location of these promoters is highly similar to that of the E. coli dnaK promoters. No transcriptional start sites were detected upstream of the dnaJ gene. The V. harveyi dnaKJ operon cloned in a plasmid in E. coli cells was transcribed in a sigma32 dependent manner and the size of the transcript, the kinetics of transcription, and the transcriptional start sites were as in V. harveyi cells. This indicates a high conservation of the transcriptional heat shock regulatory elements between E. coli and V. harveyi, both belonging to the gamma subdivision of proteobacteria. We tested the ability of the cloned dnaKJ genes to complement E. coli dnaK and dnaJ mutants and found that V. harveyi DnaJ restored a thermoresistant phenotype to dnaJ mutants and enabled lambda phage to grow in the mutant cells. V. harveyi DnaK did not suppress the thermosensitivity of dnaK mutants but complemented the dnaK deletion mutant with respect to growth of lambda phage. V. harveyi DnaK, in contrast to DnaJ, failed to modulate the heat shock response in E. coli. Our results suggest that the DnaK chaperone may be more species specific than the DnaJ chaperone.