The biosynthetic origin of the pyridone ring of efrotomycin

Summary Nocardialactamdurans has been shown to catabolyse uracil via the reductive pathway. The end product of this pathway, -alanine, is incorporated into the pyridone ring of efrotomycin. Support for this proposal includes: (1) reversal of thymine inhibition of efrotomycin biosynthesis by dihydrouracil andN-carbamoyl--aline, two intermediates of the catabolic pathway; (2) incorporation of 5,6-³H]-uracil into efrotomycin with a relative molar specific activity of approximately 0.5, close to the theoretical maximum; and (3)¹³C coupling at C₄ and C₅ of efrotomycin after feeding resting cells with 4,5-¹³C]-uracil. Our results do not rule out the possibility of an alternative source of -alanine or the coexistence of uracil catabolism via oxidative reactions.