Abstract
Water soluble extracts of Escherichia coli cells have been found to exert an extremely strong repressive effect upon the expression of catabolite sensitive operons. The compound responsible for this activity has been partially purified and proves to be of low molecular weight and heat stable. The effect of this compound, hereafter designated as catabolite modulator factor, is only partially antagonized by adenosine 3':5'-cyclic monophosphate. The possible role of catabolite modulator factor in the physiological regulation of catabolite repression is discussed.