An analogue of the DnaJ molecular chaperone in Escherichia coli

Escherichia coli DnaJ functions as a typical molecular chaperone in coordination with other heat shock proteins such as DnaK and GrpE in a variety of cellular processes. In this study, it was found that E. coli possesses an analogue of DnaJ, as judged from not only its primary structure but also its possible function. This protein, named CbpA (for curved DNA-binding protein), was first identified as a DNA-binding protein that preferentially recognizes a curved DNA sequence. Cloning and nucleotide sequencing of the gene encoding CbpA revealed that the predicted product is very similar to DnaJ in amino acid sequence: overall identity is 39%. The cbpA gene functions as a multicopy suppressor for dnaJ mutations. The mutational lesions characteristic of a dnaJ null mutant--namely, temperature sensitivity for growth and defects in lambda phage and mini-F DNA replication--were all restored upon introduction of the cbpA gene on a multicopy plasmid. An insertional mutant of cbpA was also isolated, which showed no noticeable phenotype, particularly with regard to temperature sensitivity for growth. However, when this cbpA::kan allele was combined with the dnaJ null allele, the resultant strain was unable to grow at 37 degrees C, at which strains carrying each mutation alone could grow normally. These genetic results are interpreted as meaning that the function(s) of CbpA in E. coli is closely related to that of DnaJ.