AN ANALOG OF THE DNAJ MOLECULAR CHAPERONE IN ESCHERICHIA-COLI

Escherichia coli DnaJ functions as a typical molecular chaperone in co ordination with other heat shock proteins such as DnaK and GrpE in a v ariety of cellular processes. In this study, it was found that E. coli possesses an analogue of DnaJ, as judged from not only its primary st ructure but also its possible function. This protein, named CbpA (for curved DNA-binding protein), was first identified as a DNA-binding pro tein that preferentially recognizes a curved DNA sequence. Cloning and nucleotide sequencing of the gene encoding CbpA revealed that the pre dicted product is very similar to DnaJ in amino acid sequence: overall identity is 39%. The cbpA gene functions as a multicopy suppressor fo r dnaJ mutations. The mutational lesions characteristic of a dnaJ null mutant-namely, temperature sensitivity for growth and defects in A ph age and mini-F DNA replication-were all restored upon introduction of the cbpA gene on a multicopy plasmid. An insertional mutant of cbpA wa s also isolated, which showed no noticeable phenotype, particularly wi th regard to temperature sensitivity for growth. However, when this cb pA=kan allele was combined with the dnaJ null allele, the resultant st rain was unable to grow at 37-degrees-C, at which strains carrying eac h mutation alone could grow normally. These genetic results are interp reted as meaning that the function(s) of CbpA in E. coli is closely re lated to that of DnaJ.