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Arsenate Reductase (ArsC) family, Spx subfamily; Spx is a unique RNA polymerase (RNAP)-binding protein present in bacilli and some mollicutes. It inhibits transcription by binding to the C-terminal domain of the alpha subunit of RNAP, disrupting complex formation between RNAP and certain transcriptional activator proteins like ResD and ComA. In response to oxidative stress, Spx can also activate transcription, making it a general regulator that exerts both positive and negative control over transcription initiation. Spx has been shown to exert redox-sensitive transcriptional control over genes like trxA (TRX) and trxB (TRX reductase), genes that function in thiol homeostasis. This redox-sensitive activity is dependent on the presence of a CXXC motif, present in some members of the Spx subfamily, that acts as a thiol/disulfide switch. Spx has also been shown to repress genes in a sulfate-dependent manner independent of the presence of the CXXC motif.
Jiyao W et al.(2023). "The conserved domain database in 2023.",