CysB is a member of the LysR family of transcriptional activators and regulates genes of the cysteine regulon in Salmonella typhimurium and Escherichia coli. CysB binds to specific sites just upstream of the -35 regions of the cysJIH, cysK, and cysP promoters, where, in the presence of N-acetyl-L-serine, it stimulates transcription initiation. The cysK and cysP promoters contain additional binding sites, and we have proposed that CysB bends these promoters by binding to adjacent sites. N-Acetyl-L-serine is thought to decrease the magnitude of such bending. Since stoichiometric data bearing on this model have been lacking, we analyzed complexes in gel mobility shift experiments with 35S-labeled CysB and 32P-labeled promoter fragments. CysB was found to bind as a tetramer, and N-acetyl-L-serine increased the electrophoretic mobilities of one-protein complexes of the multibinding site cysK and cysP promoters without changing their stoichiometry, indicating that a single CysB tetramer can bend these promoters and that N-acetyl-L-serine diminishes such bending. Bend angles for both promoters were calculated to be 100 and 50 degrees in the absence and presence of N-acetyl-L-serine. N-Acetyl-L-serine affected neither the stoichiometry nor the electrophoretic mobility of cysJIH promoter complexes, which are not known to contain bent DNA. DNA bending may be a mechanism for sequestering CysB at certain promoter sites by increasing their affinity for this protein in the absence of N-acetyl-L-serine.