The biosynthetic origin of the pyridone ring of efrotomycin.

Nocardia lactamdurans has been shown to catabolyse uracil via the reductive pathway. The end product of this pathway, beta-alanine, is incorporated into the pyridone ring of efrotomycin. Support for this proposal includes: (1) reversal of thymine inhibition of efrotomycin biosynthesis by dihydrouracil and N-carbamoyl-beta-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 beta-alanine or the coexistence of uracil catabolism via oxidative reactions.