Summary
6-Azauracil is a competitive antagonist of uracil and uridine in a strain of Lactobacillus bulgaricus, 09-X, able to utilize uracil for growth, and a noncompetitive inhibitor of growth on orotate in this and the parent strain, L. bulgaricus 09-1. The growth of a uracil-requiring mutant of E. coli, Wc—, is essentially unaffected by azauracil. Pediococcus cerevisiae (Leuconostoc citrovorum), grown either on thymidine or synthetic citrovorum factor; L. casei, grown either on thymine or pteroylglutamic acid (PGA); and L. leichmannii, grown either on thymidine or vitamin B12, with or without uracil, are inhibited strongly by azauracil; the inhibition can be reversed in all cases by excess uracil. Streptococcus faecalis grown on thymine, thymidine, or PGA is inhibited by azauracil. In the absense of uracil, a population evolves which is resistant to azauracil; however, in the presence of a low level of uracil and a low but inhibitory concentration of azauracil, an antagonist-resistant strain does not emerge. The degree of inhibition caused by azauracil is greater at a supra-optimal, as contrasted with a limiting, level of growth factor (PGA, thymine, or thymidine). Resting cells of S. faecalis incubated with thymidine and azauracil accumulate a compound with the properties of the deoxyriboside of azauracil. Possible mechanisms of action and the significance of the inhibition patterns are discussed.