The strong overexpression of heterologous genes in Escherichia coli often leads to inhibition of cell growth, ribosome destruction, loss of culturability, and induction of stress responses, such as a heat shock-like response. Here we demonstrate that the general stress response, which is connected to the stress response regulator sigmas (sigma38, rpoS gene product), is suppressed during strong overproduction of a heterologous alpha-glucosidase. The mRNA levels of the rpoS and osmY stress genes drastically decrease after induction of the strong overexpression system. It is shown that an rpoS mutation causes a significant loss of cell viability after induction of the expression system. Furthermore, it is demonstrated that an E. coli c/pP mutant, which could be suggested to improve heterologous protein production, is not a good production host if a tac-promoter is used to control the expression of the recombinant gene. Data from this study suggest that the overexpression of the alpha-glucosidase was greatly decreased by sigma factor competition in the clpP mutant, due to the increased sigmas level in this mutant background.
