AraC Functional Suppressors of Mutations in the C-Terminal Domain of the RpoA Subunit of the RNA Polymerase.

In , transcriptional activation is often mediated by the C-terminal domain of RpoA, the α subunit of RNA polymerase. Random mutations that prevent activation of the arabinose P promoter are clustered in the RpoA C-terminal domain (α-CTD). We have isolated functional suppressors of α-CTD mutations that map to , the main transcriptional regulator of genes, or to the P promoter. No mutation was found in the DNA regulatory region between and P. Most suppressors that improve P transcription are localized to the N-terminal domain of AraC. One class of mutations generates substitutions in the core of the N-terminal domain, suggesting that they affect its conformation. Other suppressors localize to the flexible N-terminal arm of AraC. Some, but not all, suppressors confer an arabinose constitutive phenotype. Suppression by both classes of mutations requires the α-CTD to stimulate expression from P. Surprisingly, in strains lacking Crp, the cAMP receptor protein, these mutations largely restore arabinose gene expression and can essentially bypass Crp activation. Thus, the N-terminal domain of AraC exhibits at least three distinct activities: dimerization, arabinose binding, and transcriptional activation. Finally, one mutation maps to the AraC C-terminal domain and can synergize with AraC mutations in the N-terminal domain.