Preparation for publication of a large backlog of research findings has been completed. The physiological effects of the potent inhibitions of threonine synthase by Pi and AMP, and of cystathionine Gamma-synthase by Pi, were examined. Each of the inhibitions was competitive with O-phosphohomoserine. Tissue Pi and AMP were determined, and concentrations estimated for chloroplasts, the organelle in which threonine synthase and cystathionine Gamma-synthase are localized. Calculations indicated that for growth at standard external Pi or above, if the substrates O-phosphohomoserine, cysteine) and the effector S-adenosylmethionine were uniformly distributed within plants, total activities of the two enzymes in question would fall two orders of magnitude below the amounts required to provide threonine and methionine adequate for growth. One factor, namely an increase in the concentration of 0-phosphohomoserine, has the potential to restore the activities of both enzymes to meet physiological needs. If 0-phosphohomoserine were restricted to chloroplasts, these inhibitions would be much less severe, resulting in activities approaching the required physiological amounts. Exploratory studies were initiated to elucidate the in vivo regulatory patterns of threonine biosynthesis. Threonine synthesis appears to be at least partially regulated. Regulation of isoleucine synthesis was demonstrated at a site or sites distal to threonine. Plants supplemented with methionine contained normal concentrations of soluble threonine, arguing against the proposal that an increased concentration of soluble threonine is a key step in regulation of methionine biosynthesis, and further suggesting that methionine may regulate synthesis of 0-phosphohoserine in addition to its established control at cystathionine Gamma-synthase.
