Purification of the cysB protein from Salmonella typhimurium

Using an assay based on polypeptide mobility in one- and two-dimensional polyacrylamide electropherograms, we purified the Salmonella typhimurium cysB protein from an Escherichia coli strain carrying plasmid pGBK14, in which the S. typhimurium cysB gene is under transcriptional control of the strong promoter PL from phage lambda. cysB protein constitutes approximately 4% of total soluble protein in such cells and was obtained with good yield at a purity of 85% after precipitation of nucleic acids with streptomycin sulfate, ammonium sulfate fractionation, and hydrophobic chromatography on methyl agarose. Material with 95% purity was obtained with poor yield by ion-exchange chromatography of native protein and with good yield by size exclusion chromatography of denatured protein in sodium dodecyl sulfate. Physicochemical studies of the purified cysB protein show that it is a tetramer of identical 36-kDa subunits with a pI, amino acid composition, amino-terminal sequence, and carboxyl-terminal amino acid content identical to those known from previous studies or deduced from the cysB DNA sequence. Although O-acetyl-L-serine is believed to bind to cysB protein to form a complex that stimulates transcription of genes of the cysteine regulon, we were unable to demonstrate such an interaction using methods that would have detected binding with a Kd of less than 0.1 mM.