Induced Levels of Heat Shock Proteins in dnaK mutants of Lactococcus lactis

The bacterial heat shock response is characterized by the elevated expression of a number of chaperone complexes and proteases including the DnaK-GrpE-DnaJ and the GroELS chaperone complexes. In order to investigate the importance of the DnaK chaperone complex for the growth and the heat shock regulation in Lactococcus lactis we have constructed two dnaK mutants with C-terminal deletions in dnaK. The minor deletion of 65 amino acids in the dnaK2 mutant, results in a slightly temperature sensitive phenotype. BK6 containing the larger deletion of 174 amino acids (dnaK1) removing the major part of the inferred substrate binding site of the DnaK protein, exhibits a pronounced temperature sensitive phenotype and shows altered regulation of the heat shock response. The expression of the heat shock proteins are increased at the normal growth temperature measured both as protein synthesis rates and mRNA levels which indicate that DnaK could be involved in the regulation of the heat shock response in Lactococcus lactis. In Bacillus subtilis it has been found by Mogk,A., G.Homouth, C. Scholz, L. Kim, F.X. Schmid, and W. Schumann. 1997. EMBO J. 16: 4579-4590, that the activity of the heat shock repressor, HrcA , is dependent of the chaperone function of the GroELS complex and that an insertion mutant in dnaK did not have any effect on the expression of the heat shock proteins. The present data from Lactococcus lactis suggest that the DnaK protein could be involved in maturation of the homologous HrcA protein in this bacterium.