Microbiological assay for thymine using a mutant of Streptococcus faecalis.

Doctor, V. M. (University of Texas, Houston). Microbiological assay for thymine using a mutant of Streptococcus faecalis. J. Bacteriol. 84:17-22. 1962.-A rapid microbiological assay procedure for the determination of thymine was devised. The test organism, Streptococcus faecalis Akt, a mutant of S. faecalis ATCC 8043, gives one-half the maximal growth response to 1.6 mug of thymine (assay range 0.5 to 2.5 mug) or to a molar equivalent (3.2 mug) of thymidine. Under these experimental conditions, 80 mug of 5-methylcytosine or 150 mug of thymidylic acid replace the thymine or thymidine requirement of the test organism. Addition of a mixture of adenine, guanine, and xanthine at a level of 40 mug/ml of each or higher to the basal medium, or addition of a mixture of deoxyadenosine and deoxyguanosine at a level of 80 mug/ml of each or higher to the basal medium, inhibited the thymine or thymidine growth response of the organism. Thymine activities of rat and chick tissues were determined by this procedure. The data on the recovery of added thymine to tissues gave 84 to 103% recovery. Thymus, spleen, small intestine, and ventral prostate were higher in thymine activities as compared with other tissues.