A novel DNA element that controls bacterial heat shock gene expression

Abstract

The hspArpoH1 and hspBCdegP heat shock operons of Bradyrhizobium japonicum are preceded by a novel, conserved DNA element of approximately 100 bp, which is responsible for the temperature-regulated transcription of their sigma70-type promoters. We designated this motif ROSE for repression of heat shock gene expression and found additional ROSE elements upstream of two newly identified heat shock operons. A critical core region in the hspA-associated ROSE1 was defined by introducing insertions or deletions. While four mutants retained the ability to repress transcription of the hspArpoH1 operon, five deletion mutants produced elevated hspA mRNA levels under low-temperature growth conditions. Derepression was confirmed by increased RpoH1 levels in non-heat-shocked cells from one of these mutants and by strains that contained a translational hspA-lacZ fusion associated with mutated ROSE1 elements. The hspArpoH1 operon was efficiently transcribed in vitro, and a deletion of ROSE1 did not impair this activity. Gel retardation experiments demonstrated that a protein in non-heat-shocked cells specifically binds to the intact ROSE1 element but not to a mutated element lacking the core region. Taken together, these results indicate that a central region of ROSE serves as a binding site for a repressor protein under standard growth conditions in order to prevent the undesired transcription of heat shock genes.