Heat-induced expression and chemically induced expression of the Escherichia coli stress protein HtpG are affected by the growth environment

Differences in expression of the Escherichia coli stress protein HtpG were found following exposure of exponentially growing cells to heat or chemical shock when cells were growth under different environmental conditions. Wit h an htpG::lacZ reporter system, htpG expression increased in cells grown i n a complex medium (Luria-Bertani [LB] broth) following a temperature shock at 45 degrees C. In contrast, no HtpG overexpression was detected in cells grown in a glucose minimal medium, despite a decrease in the growth rate. Similarly, in pyruvate-grown cells there was no heat shock induction of Htp G expression, eliminating the possibility that repression of HtpG in glucos e-grown E. coli was due to catabolite repression. When 5 mM phenol was used as a chemical stress agent for cells growing in LB broth, expression of Ht pG increased. However, when LB-grown cells were subjected to stress with 10 mM phenol and when both 5 and 10 mM phenol were added to glucose-grown cul tures, repression of htpG expression was observed. 2-Chlorophenol stress re sulted in overexpression of HtpG when cells were grown in complex medium bu t repression of HtpG synthesis when cells were grown in glucose. No inducti on of htpG expression was seen,vith 2,4-dichlorophenol in cells grown with either complex medium or glucose. The results suggest that, when a large po ol of amino acids and proteins is available, as in complex medium, a much s tronger stress response is observed. In contrast, when cells are grown in a simple glucose mineral medium, htpG expression either is unaffected or is even repressed by imposition of a stress condition. The results demonstrate the importance of considering differences in growth environment in order t o better understand the nature of the response to an imposed stress conditi on.