Spx is a global transcriptional regulator that is conserved among Gram-positive bacteria, in which Spx is required for preventing oxidatively induced proteotoxicity. Upon stress induction, Spx engages RNA polymerase (RNAP) through interaction with the C-terminal domain of the -encoded RNAP α subunit (αCTD). Previous mutational analysis of revealed that substitutions of Y263 in αCTD severely impaired Spx-activated transcription. Attempts to substitute alanine for αCTD R261, R268, R289, E255, E298, and K294 were unsuccessful, suggesting that these residues are essential. To determine whether these RpoA residues were required for productive Spx-RNAP interaction, we ectopically expressed the putatively lethal mutant alleles in the mutant, where " " indicates the amino acid change that results from mutation of the allele. By complementation analysis, we show that Spx-bound αCTD amino acid residues are not essential for Spx-activated transcription but that R261A, E298A, and E255A mutants confer a partial defect in NaCl-stress induction of Spx-controlled genes. In addition, strains expressing are defective in disulfide stress resistance and produce RNAP having a reduced affinity for Spx. The E255 residue corresponds to αD259, which has been implicated in αCTD-σ interaction (σ R603, corresponding to R362 of σ ). However, the combined and mutations have an additive negative effect on Spx-dependent expression, suggesting the residues' differing roles in Spx-activated transcription. Our findings suggest that, while αCTD is essential for Spx-activated transcription, Spx is the primary DNA-binding determinant of the Spx-αCTD complex. Though extensively studied in , the role of αCTD in activator-stimulated transcription is largely uncharacterized in Here, we conduct phenotypic analyses of putatively lethal αCTD alanine codon substitution mutants to determine whether these residues function in specific DNA binding at the Spx-αCTD-DNA interface. Our findings suggest that multisubunit RNAP contact to Spx is optimal for activation while Spx fulfills the most stringent requirement of upstream promoter binding. Furthermore, several αCTD residues targeted for mutagenesis in this study are conserved among many bacterial species and thus insights on their function in other regulatory systems may be suggested herein.