CysB in the Multiverse of Functions: Regulatory Roles in Cysteine Biosynthesis and Beyond.

CysB is a member of the large bacterial LysR-type transcriptional regulator (LTTR) protein family. Like the majority of LTTRs, CysB functions as a homotetramer in which each subunit has an N-terminal winged-helix-turn-helix (wHTH) DNA-binding domain connected to an effector-binding domain by a helical hinge region. CysB is best known for its role in regulating the expression of genes associated with sulfur uptake and biosynthesis of cysteine in Gram-negative species such as Escherichia coli and Salmonella enterica. Activation of CysB target genes generally requires the effector N-acetyl-L-serine, which derives from an intermediate in the cysteine biosynthetic pathway. Here, we outline the established roles of CysB in controlling the cysteine regulon, complemented with an interpretation of DNA binding modes inspired by the recently published structure of full-length CysB that is consistent with the 'sliding dimer' model proposed for many LTTRs. Notably, CysB orthologs have been described for which N-acetyl-L-serine does not appear to be required as an effector, and CysB regulons frequently include genes that are not directly related to sulfur assimilation and cysteine biosynthesis. Examples include hslJ, which encodes a predicted membrane protein involved in novobiocin resistance in E. coli, and pqsR, encoding a transcriptional regulator involved in Pseudomonas Quinolone Signal production and virulence in Pseudomonas aeruginosa. These data suggest that CysB orthologs have diverged to ensure optimal function and incorporation in distinct gene regulatory networks.