The bacterial heat shock proteins DnaK and DnaJ are members of a class of molecular chaperones that are required for a wide variety of cellular functions at normal growth temperatures. In Caulobacter crescentus, the expression of the dnaKJ operon is regulated both temporally during the normal cell cycle and by heat shock. Analysis of deletions and base substitutions in the 5' region of the operon established the presence of two functional promoters: a heat shock-inducible promoter, P1, with characteristics of a sigma 32 promoter, and an adjacent sigma 70-like promoter, P2. Transcription initiating at the sigma 70-like promoter is under strict temporal control, whereas transcription initiating at the heat shock promoter at 30 degrees C is not. Transcription of dnaKJ occurs during a short period in the cell cycle, concomitant with the onset of DNA replication. Deletions in the 5' region have also revealed that all cis-acting sites required for temporal control of transcription reside within 50 bases of the P2 start site. Transcripts initiating from either the P1 or the P2 promoter have an RNA leader sequence with a high probability of forming an extensive secondary structure. Deletion of this leader sequence resulted in an increased rate of expression in both transcriptional and translational fusions. Although the temporal control of expression at physiological temperatures is not affected by the presence or absence of the leader sequence, changes in mRNA secondary structure may contribute to the modulation of DnaK and DnaJ levels at normal temperatures and during heat shock.