Global transcriptional regulator that plays a key role in stress response and exerts either positive or negative regulation of genes. Acts by interacting with the C-terminal domain of the alpha subunit of the RNA polymerase (RNAP). This interaction can enhance binding of RNAP to the promoter region of target genes and stimulate their transcription, or block interaction of RNAP with activator. Interacts with the C-terminal domain of the alpha subunit of the RNAP. Belongs to the ArsC family. Spx subfamily.