DnaK is a major heat shock protein of Escherichia coli and has been previously reported to be essential for growth at high temperatures. We systematically investigated the role of DnaK in cellular metabolism at a wide range of growth temperatures by analyzing cellular defects caused by deletion of the dnaK gene (delta dnaK52). At intermediate temperatures (30 degrees C), introduction of the delta dnaK52 allele into wild-type cells caused severe defects in cell division, slow growth, and poor viability of the cells. delta dnaK52 mutants were genetically unstable at 30 degrees C and frequently acquired secondary mutations. At high (42 degrees C) and low (11 and 16 degrees C) temperatures the delta dnaK52 allele could only be introduced into the subpopulation of wild-type cells that had duplicated the dnaK region of their chromosome. delta dnaK52 mutants isolated at 30 degrees C were cold sensitive as well as temperature sensitive for growth. Cell division defects of delta dnaK52 mutants at 30 degrees C were largely suppressed by overproduction of the FtsZ protein, which is normally required for septation during cell division; however, slow growth and poor viability at 30 degrees C and cold sensitivity and temperature sensitivity of growth were not suppressed, indicating that delta dnaK52 mutants had additional defective cellular functions besides cell division.