Ambidextrous transcriptional activation by SoxS: requirement for the C-terminal domain of the RNA polymerase alpha subunit in a subset of Escherichia coli superoxide-inducible genes.

Purified MalE-SoxS fusion protein specifically stimulated in vitro transcription of the Escherichia coli zwf, fpr, fumC, micF, nfo, and sodA genes, indicating that activation of the superoxide regulon requires only SoxS. As in vivo, a 21 bp sequence adjacent to the zwf promoter was able to activate transcription of an heterologous promoter in vitro. Activation of zwf and fpr transcription required the C-terminal domain (CTD) of the RNA polymerase alpha subunit, while stimulation of fumC, micF, nfo, and sodA transcription was independent of CTD truncation. Thus, like the catabolite gene activator protein (CAP), SoxS is an 'ambidextrous' activator, activation only requiring the alpha CTD in a subset of regulated promoters. Indeed, the -35 hexamers of the zwf and fpr promoters lie downstream of the respective MalE-SoxS binding sites, while the binding sites of fumC, micF, nfo, and sodA overlap their -35 promoter hexamers.