Systematic mutagenesis of the DNA binding sites for SoxS in the Escherichia coli zwf and fpr promoters: identifying nucleotides required for DNA binding and transcription activation.

SoxS is the direct transcriptional activator of at least 15 genes of the Escherichia coli superoxide regulon. SoxS is small (107 amino acids), binds DNA as a monomer and recognizes a highly degenerate DNA binding site, termed 'soxbox'. Like other members of the AraC/XylS family, SoxS has two putative helix-turn-helix (HTH) DNA-binding motifs, and it has been proposed that each HTH motif recognizes a highly conserved recognition element of the soxbox. To determine which nucleotides are important for SoxS binding, we conducted a systematic mutagenesis of the DNA binding sites for SoxS in the zwf and fpr promoters and determined the effect of the soxbox mutations on SoxS DNA binding and transcription activation in vivo by measuring beta-galactosidase activity in strains with fusions to lacZ. We found that the sequences GCAC and CAAA, termed recognition elements 1 and 2 (RE 1 and RE 2), respectively, are critical for SoxS binding, as mutations within these elements severely hinder or eliminate SoxS-dependent transcription activation; substitutions within RE 2 (CAAA), however, are tolerated better than changes within RE 1 (GCAC). Although substitutions at the seven positions separating the two REs had only a modest effect on SoxS binding, AT basepairs were favoured within this 'spacer' region, presumably because, by facilitating DNA bending, they help bring the two recognition elements into proper juxtaposition. We also found that the 'invariant A' present at position 1 of 14/15 functional soxboxes identified thus far is important for SoxS binding, as a change to any other nucleotide at this position reduced SoxS-dependent transcription by approximately 50%. In addition, positions surrounding the REs seem to show a context effect, in that certain substitutions there have little or no effect when the RE has the optimal binding sequence, but produce a pronounced effect when the RE has a suboptimal sequence. We propose that these nucleotides play an important role in effecting differential expression from the various promoters. Lastly, we used gel retardation assays to show that alterations in transcription activation in vivo are caused by effects on DNA binding. Based on this exhaustive mutagenesis, we propose the following optimal sequence for SoxS binding: AnVGCACWWWnKRHCAAAHn (n = A, C, G, T; V = A, C, G; W = A, T; K = G, T; R = A, G; H = A, C, T).