Heat shock protein GroE of Escherichia coli: key protective roles against thermal stress.

An Escherichia coli mutant lacking the heat shock sigma-factor (sigma 32) is defective in transcription from heat shock promoters and cannot grow at temperatures above 20 degrees C. To assess physiological roles of sigma 32 and heat shock proteins, we isolated and characterized a set of temperature-resistant revertants from this deletion (delta rpoH) mutant. Most of them were found to carry a DNA insertion in the groE upstream region, resulting in high-level synthesis of major heat shock proteins GroE (GroES and GroEL). The levels of GroE produced varied in different revertants and correlated well with the maximum permissive temperatures; the highest GroE producers (approximately 10% of total protein) grew up to 40 degrees C but not at 42 degrees C. An additional mutation causing hyperproduction of DnaK (hsp70 homolog) was required for growth at 42 degrees C. Such effects of GroE and DnaK on the sigma 32-deletion strains were also confirmed by using multicopy plasmids carrying groE or dnaK. Thus, GroE plays a key protective role in supporting growth at normal physiological temperatures (20-40 degrees C), whereas high levels of DnaK are required primarily at higher temperature.