The activity of sigma 32 is reduced under conditions of excess heat shock protein production in Escherichia coli

Abstract

The expression of heat shock genes in Escherichia coli is controlled by the action of an alternate sigma-factor of RNA polymerase, sigma 32, which directs core RNA polymerase to recognize the promoters for heat shock genes. After a shift from 30 degrees C to 42 degrees C, both the level of sigma 32 and transcription initiation at heat shock promoters transiently increase, indicating that heat shock gene expression is regulated by changes in the concentration of sigma 32. Here, we report that heat shock gene expression is regulated by changes in the activity of sigma 32 under some conditions. Our results show that the transient repression of heat shock protein synthesis, which follows a shift down from 42 degrees C to 30 degrees, occurs as a result of decreased transcription initiation at heat shock promoters, but this repression is accompanied by only a small decrease in the level of sigma 32. In addition, the induction of heat shock proteins following overproduction of sigma 32 from a multicopy plasmid is only transient, despite the fact that the level of sigma 32 remains elevated. Constitutive overproduction of sigma 32 also fails to cause a proportionate increase in heat shock gene transcription. These three examples suggest that the activity of sigma 32 is reduced under conditions of excess heat shock gene expression.