Interacts with DnaK and GrpE to disassemble a protein complex at the origins of replication of phage lambda and several plasmids. Participates actively in the response to hyperosmotic and heat shock by preventing the aggregation of stress-denatured proteins and by disaggregating proteins, also in an autonomous, DnaK-independent fashion. Unfolded proteins bind initially to DnaJ; upon interaction with the DnaJ-bound protein, DnaK hydrolyzes its bound ATP, resulting in the formation of a stable complex. GrpE releases ADP from DnaK; ATP binding to DnaK triggers the release of the substrate protein, thus completing the reaction cycle. Several rounds of ATP-dependent interactions between DnaJ, DnaK and GrpE are required for fully efficient folding. Binds 2 Zn(2+) ions per monomer. Homodimer. By heat shock under the control of the HtpR regulatory protein. The J domain is necessary and sufficient to stimulate DnaK ATPase activity. Zinc center 1 plays an important role in the autonomous, DnaK-independent chaperone activity of DnaJ. Zinc center 2 is essential for interaction with DnaK and for DnaJ activity. Single dnaJ and double dnaK-dnaJ disruption are non-essential; synthetic lethality is seen in a triple tig-dnaK-dnaJ disruption, although this depends on temperature (triple disruptions grow slowly at 20 and 34 degrees Celsius but not at 43 degrees) and strain background. Belongs to the DnaJ family.