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Background  Spx, an ArsC (arsenate reductase) family member, is a global transcriptional regulator of the microbial stress response and is highly conserved amongst Gram-positive bacteria.  Bacillus subtilis  Spx protein exerts positive and negative control of transcription through its interaction with the C-terminal domain of the RNA polymerase (RNAP) α subunit (αCTD). Spx activates  trxA  (thioredoxin) and  trxB  (thioredoxin reductase) in response to thiol stress, and bears an N-terminal C10XXC13 redox disulfide center that is oxidized in active Spx.    Methodology/Principal Findings  The structure of mutant Spx C10S  showed a change in the conformation of helix α4. Amino acid substitutions R60E and K62E within and adjacent to helix α4 conferred defects in Spx-activated transcription but not Spx-dependent repression. Electrophoretic mobility-shift assays showed αCTD interaction with  trxB  promoter DNA, but addition of Spx generated a supershifted complex that was disrupted in the presence of reductant (DTT). Interaction of αCTD/Spx complex with promoter DNA required the  cis -acting elements -45AGCA-42 and -34AGCG-31 of the  trxB  promoter. The Spx G52R  mutant, defective in αCTD binding, did not interact with the αCTD- trxB  complex. Spx R60E  not only failed to complex with αCTD- trxB , but also disrupted αCTD- trxB  DNA interaction.    Conclusions/Significance  The results show that Spx and αCTD form a complex that recognizes the promoter DNA of an Spx-controlled gene. A conformational change during oxidation of Spx to the disulfide form likely alters the structure of Spx α helix α4, which contains residues that function in transcriptional activation and αCTD/Spx-promoter interaction. The results suggest that one of these residues, R60 of the α4 region of oxidized Spx, functions in αCTD/Spx-promoter contact but not in αCTD interaction.

Promoter Recognition by a Complex of Spx and the C-Terminal Domain of the RNA Polymerase α Subunit