Leavitt, Richard I. (Long Island Biological Association, Cold Spring Harbor, N.Y.). Isoleucine and valine metabolism in Escherichia coli . XII. Characterization of a factor involved in acetolactate formation. J. Bacteriol. 88: 172–178. 1964.—An unknown factor required for the formation of acetolactate, the first five-carbon precursor of valine, is described. The presence of this factor eliminates the dilution effect encountered when studying the effect of increasing enzyme concentration on the formation of acetolactate. It is suggested that this factor functions by increasing the enzyme affinities of both pyruvate and thiamine pyrophosphate.