Master repressor of cysteine metabolism in B.subtilis. Controls the expression of genes involved either in cysteine synthesis from sulfide (cysK), sulfonates (ssu), or methionine (mccAB) or in cystine uptake (tcyP). Activity of CymR is positively regulated by CysK in response to cysteine availability. When cysteine is present, the pool of O-acetylserine (OAS) is low, which leads to the formation of a CymR-CysK complex and transcriptional repression of the CymR regulon occurs. In the absence of cysteine, the OAS pool is high and the CymR-CysK complex is mostly dissociated, leading to a faster dissociation of CymR from its DNA targets and the lifting of CymR-dependent repression. Homodimer. Forms homotetramers at higher concentrations of protein. Forms CymR(2):CysK(2) or CymR(4):CysK(4) complexes in the absence of O-acetylserine. Grows slowly with cystine as sole sulfur source.