Fig. 1.

The effects of single alanine substitutions of σ⁷⁰ on Rob-dependent transcription activation of the class II fumC and micF promoters. Derivatives of strain RA4468 containing single-copy λ prophages carrying transcriptional fusions of lac to the fumC (a) and micF (b) promoters and harboring plasmid pBAD33-Rob were transformed with compatible plasmids encoding wild-type σ⁷⁰ or mutant alleles of σ⁷⁰ with single alanine substitutions at the indicated positions. Determination of the activity of β-galactosidase in the respective strains was carried out by themethod of Griffith and Wolf,⁴⁴ as described in Materials and Methods. The β-galactosidase activity produced by each mutant strain is given as a percentage of the activity produced by the strain expressing wild-type σ⁷⁰. Activity values at or below 80% of the activity produced by wild-type σ⁷⁰, denoted by the horizontal line, are considered to be significant, as discussed in the text.