Induction of a heat shock-like response by unfolded protein in Escherichia coli: dependence on protein level not protein degradation.

To test the idea that unfolded protein might act as an intracellular signal for induction of the heat shock response in Escherichia coli, we examined the synthesis of several heat shock proteins after expression of an unfolded variant of the amino-terminal domain of lambda repressor. These experiments show that expression of a single mutant protein, and not its wild-type counterpart, is sufficient to induce a heat shock-like response. In addition, by measuring the abilities of unfolded variants of differing proteolytic susceptibilities to induce heat shock protein synthesis and by monitoring heat shock protein synthesis as a function of the amount of a single unfolded protein, we show that it is the concentration of unfolded protein in the cell, and not its degradation, that is important for inducing the heat shock-like response.