Isolation of a second rifamycin-binding from Escherichia coli by affinity chromatography

When extracts of Escherichia coli are filtered through a Sepharose column containing covalently bound rifamycin a protein is bound which can be eluted either with a high concentration of urea or more specifically with low concentrations of rifamycins. Its Mr is 18,000 +/- 1,000 in the presence of dodecylsulfate, in its absence 36,000 +/- 3,000. The association constant of the protein for rifampicin is 2.4 +/- 0.5 x 10(-4) M with two binding sites per dimer as determined by equilibrium dialysis. Large amounts of this protein are released from the cells by an osmotic shock.