Mutational analysis of a bacterial promoter. The Lac promoter, driving the lac operon, has a sequence close to the σ70 consensus sequence. (a) Mutations are created that make the promoter conform to the consensus; red dashes indicate lack of a nucleotide at that position. The spacer length matters because it governs the helical orientation of the two consensus sites. (b) Mutation results in increased expression and activity of β-galactosidase.