Induction of heat shock proteins by abnormal proteins results from stabilization and not increased synthesis of σ32 in Escherichia coli

Abstract

Accumulation of abnormal proteins in cells of bacteria or eukaryotes can induce synthesis of a set of heat shock proteins. We examined such induction following addition of azetidine (a proline analog) or synthesis of a heterologous protein (human prourokinase) in Escherichia coli. Synthesis of heat shock proteins under these conditions increased almost immediately and continued with increasing rates until it reached a maximum after 30 to 60 min at 30°C. The induction was closely accompanied by an increase in the cellular level of σ32 specifically required for transcription of heat shock genes. The increase in σ32 initially coincided with increased synthesis of heat shock proteins but then exceeded the latter, particularly following accumulation of prourokinase. The σ32 level increase upon either treatment was found to result solely from stabilization of σ32, which is ordinarily very unstable, and not from increased synthesis of σ32. This is in contrast to what had been found when cells were exposed to a higher temperature, at which both increased synthesis and stabilization of σ32 contributed to the increased σ32 level. On the basis of these and other findings, we propose that abnormal proteins stabilize σ32 by a pathway or a mechanism distinct from that used for the induction of σ32 synthesis known to occur at the level of translation. Evidence further suggests that the DnaK chaperone plays a crucial regulatory role in induction of the heat shock response by abnormal proteins.