Transcriptional regulator that controls propionyl-CoA assimilation through the methylmalonyl-CoA pathway via regulation of pccB expression. Mutant is unable to grow with propionate-HCO(3)(-) as the carbon source. However, the mutant can grow normally with succinate or acetate as the carbon source. Belongs to the short-chain fatty acyl-CoA assimilation regulator (ScfR) family.