EXPRESSION OF THE CAULOBACTER HEAT-SHOCK GENE DNAK IS DEVELOPMENTALLY CONTROLLED DURING GROWTH AT NORMAL TEMPERATURES

Caulobacter crescentus has a single dnaK gene that is highly homologous to the hsp70 family of heat shock genes. Analysis of the cloned and sequenced dnaK gene has shown that the deduced amino acid sequence could encode a protein of 67.6 kilodaltons that is 68% identical to the DnaK protein of Escherichia coli and 49% identical to the Drosophila and human hsp70 protein family. A partial open reading frame 165 base pairs 3' to the end of dnaK encodes a peptide of 190 amino acids that is 59% identical to DnaJ of E. coli. Northern blot analysis revealed a single 4.0-kilobase mRNA homologous to the cloned fragment. Since the dnaK coding region is 1.89 kilobases, dnaK and dnaJ may be transcribed as a polycistronic message. S1 mapping and primer extension experiments showed that transcription initiated at two sites 5' to the dnaK coding sequence. A single start site of transcription was identified during heat shock at 42°C, and the predicted promoter sequence conformed to the consensus heat shock promoters of E. coli. At normal growth temperature (30°C), a different start site was identified 3' to the heat shock start site that conformed to the E. coli sigma 70 promoter consensus sequence. S1 protection assays and analysis of expression of the dnaK gene fused to the lux transcription reporter gene showed that expression of dnaK is temporally controlled under normal physiological conditions and that transcription occurs just before the initiation of DNA replication. Thus, in both human cells (I.K.L. Milarski and R.I. Morimoto, Proc. Natl. Acad. Sci. USA 83:9517-9521, 1986) and in a simple bacterium, the transcription of a hsp70 gene is temporally controlled as a function of the cell cycle under normal growth conditions.