Fig. 2.

The effects of tri-alanine substitutions of σ⁷⁰ on Rob-dependent transcription activation of one class I promoter and three class II promoters. Derivatives of strain N7840 Δ(araBAD)714 [pBAD33-his₆-Rob] harboring a member of the library of tri-alanine substitutions of σ⁷⁰⁴² carried on compatible plasmid pVR-σ and lysogenic for a single-copy λ prophage containing a transcriptional fusion of lac to a class II promoter, viz., fumC (a), inaA (b), and micF (c), or a class I promoter, viz., fpr (d), were grown as described in Materials and Methods. Rob was expressed from the arabinose-inducible P_BAD promoter, while wild-type σ⁷⁰ and σ⁷⁰ R4 mutants were expressed constitutively from a truncated galP1 promoter. Wild-type σ⁷⁰ was also constitutively expressed from chromosomal rpoD. Determination of the activity of β-galactosidase in the respective strains was carried out by the method of Griffith and Wolf,⁴⁴ as described in Materials and Methods. Activity values at or below 80% of the activity produced by wild-type σ⁷⁰, denoted by the horizontal line, are considered to be significant.