Biosynthesis of willardiine and isowillardiine in germinating pea seeds and seedlings.

The synthesis of the pyrimidinyl amino acids willardiine and isowillardiine was studied in vivo and in vitro. Uracil derivatives stimulate the biosynthesis of both compounds; the free base is the most effective. Significant incorporation of [2-(14)C]uracil and [6-(14)C]orotate into willardiine and isowillardiine was found. Incorporation of [6-(14)C]orotate was substantially decreased in the presence of uracil, and to a lesser extent by uridine and UMP. [3-(14)C]Serine was incorporated into the alanine side chain of the two uracilylalanines but not into the ring. The effect of a number of uracil analogues and inhibitors of pyrimidine metabolism was examined. Some were shown to stimulate the biosynthesis; the most noticeable effects were obtained with 6-azauracil and 2-thiouracil. Attempts to obtain extracts capable of synthesizing the uracilylalanines from uracil and serine were unsuccessful, but weak activity was observed when serine was replaced by O-acetylserine.