Amino-terminal amino acids modulate σ-factor DNA-binding activity

Abstract

Prokaryotic transcription initiation factor σ is required for sequence- specific promoter recognition by RNA polymerase. Genetic studies have indicated that σ itself interacts with DNA at the -10 and -35 promoter consensus sequences. Binding of Escherichia coli σ70 to DNA in vitro, however, can only be observed for truncated polypeptides lacking the amino- terminal amino acids. We have investigated the role of the amino terminus of E. coli σ70 in controlling DNA-binding ability. Deletion analysis indicates that amino acids within amino-terminal region 1.1 of σ70 inhibit DNA binding by the carboxy-terminal DNA-binding domains. Furthermore, inhibition of binding by the amino-terminal inhibitory domain of σ70 can be observed in trans. Likewise, the amino-terminal extensions of two alternative σ-factors, E. coli σ32 and Bacillus subtilis σ(K), negatively affect the DNA binding activity of their carboxy-terminal domains. We propose that initiation of transcription is subject to modulation as a result of the composition and/or structure of the amino terminus of the σ- subunit and that the σ family of proteins belong to a larger class of intramolecularly regulated transcriptional effectors.