The Escherichia coli DnaK chaperone machine and bacteriophage Mu late transcription

The DnaK chaperone is necessary for alpha-complementation of beta-galactosidase in Escherichia coli

We show here the involvement of the molecular chaperone DnaK from Escherichia coli in the in vivo alpha-complementation of the beta-galactosidase. In the dnaK756(Ts) mutant, alpha-complementation occurs when the organisms are grown at 30 degrees C but not at 37 or 40 degrees C, although these temperatures are permissive for bacterial growth. Plasmid-driven expression of wild-type dnaK restores the alpha-complementation in the mutant but also stimulates it in a dnaK(+) strain. In a mutant which contains a disrupted dnaK gene (DeltadnaK52::Cm(r)), alpha-complementation is also impaired, even at 30 degrees C. This observation provides an easy and original phenotype to detect subtle functional changes in a protein such as the DnaK756 chaperone, within the physiologically relevant temperature.

Induced levels of heat shock proteins in a dnaK mutant 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 growth and heat shock response 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 dnaKDelta2 mutant resulted in a slight temperature-sensitive phenotype. BK6, containing the larger deletion of 174 amino acids (dnaKDelta1), removing the major part of the inferred substrate binding site of the DnaK protein, exhibited a pronounced temperature-sensitive phenotype and showed altered regulation of the heat shock response. The expression of the heat shock proteins was increased at the normal growth temperature, measured as both protein synthesis rates and mRNA levels, indicating that DnaK could be involved in the regulation of the heat shock response in L. lactis. For Bacillus subtilis, it has been found (A. Mogk, G. Homuth, C. Scholz, L. Kim, F. X. Schmid, and W. Schumann, EMBO J. 16:4579-4590, 1997) that the activity of the heat shock repressor HrcA is dependent on the chaperone function of the GroELS complex and that a dnaK insertion mutant has no effect on the expression of the heat shock proteins. The present data from L. lactis suggest that the DnaK protein could be involved in the maturation of the homologous HrcA protein in this bacterium.

The 70-kDa heat-shock protein/DnaK chaperone system is required for the productive folding of ribulose-biphosphate carboxylase subunits in Escherichia coli

We have studied the in vivo requirements of the DnaK chaperone system for the folding of recombinant ribulose-bisphosphate carboxylase/oxygenase in Escherichia coli. Expression of functional dimeric or hexadecameric ribulose-bisphosphate carboxylase from different bacterial sources (including purple bacteria and cyanobacteria) was severely impaired in E. coli dnaK, dnaJ, or grpE mutants. These enzymes were synthesized mostly in soluble, fully enzymatically active forms in wild-type E. coli cells cultured in the temperature range 20-42 degrees C, but aggregated extensively in dnaK null mutants. Co-expression of dnaK, but not groESL, markedly reduced the aggregation of ribulose-bisphosphate carboxylase subunits in dnaK null mutants and restored the enzyme activity to levels found in isogenic wild-type strains. Ribulose-bisphosphate carboxylase expression in wild-type E. coli cells growing at 30 degrees C promoted an enhanced synthesis of stress proteins, apparently by sequestering DnaK from its negative regulatory role in this response. The overall results indicate that the DnaK chaperone system assists in vivo the folding pathway of ribulose-bisphosphate carboxylase large subunits, most probably at its very early stages.