Substitution of a Highly Conserved Histidine in the Escherichia coli Heat Shock Transcription Factor σ32 Affects Promoter Utilization In Vitro and Leads to Overexpression of the Biofilm-Associated Flu Protein In Vivo

摘要

The heat shock sigma factor (σ³² in Escherichia coli) directs the bacterial RNA polymerase to promoters of a specific sequence to form a stable complex, competent to initiate transcription of genes whose products mitigate the effects of exposure of the cell to high temperatures. The histidine at position 107 of σ³² is at the homologous position of a tryptophan residue at position 433 of the main sigma factor of E. coli, σ⁷⁰. This tryptophan is essential for the strand separation step leading to the formation of the initiation-competent RNA polymerase-promoter complex. The heat shock sigma factors of all gammaproteobacteria sequenced have a histidine at this position, while in the alpha- and deltaproteobacteria, it is a tryptophan. In vitro the alanine-for-histidine substitution at position 107 (H107A) destabilizes complexes between the GroE promoter and RNA polymerase containing σ³², implying that H107 plays a role in formation or maintenance of the strand-separated complex. In vivo, the H107A substitution in σ³² impedes recovery from heat shock (exposure to 42°C), and it also leads to overexpression at lower temperatures (30°C) of the Flu protein, which is associated with biofilm formation.