MOLECULAR AND METABOLIC CONTROL OF SECONDARY METABOLISM

Secondary metabolism is restricted to specific places in the plant. Th e concentrations of precursors and end products are the determining fa ctors in the metabolic control of synthesis and breakdown of the compo unds involved. Molecular control operates at the level of enzyme amoun t and gene expression. If the secondary product contains an element in its molecule which is derived from a mineral nutrient in the environm ent, the operation of the control mechanisms can be studied by varying the concentration of that mineral. This is exemplified by thiophene m etabolism in root cultures of Tagetes. The characteristic groups in th e molecule are two five-membered rings with a sulphur atom. In the exp eriments, the rate of thiophene biosynthesis was manipulated by varyin g the sulphate concentration in the medium. Sulphur limitation led to preferential channeling of sulphur into primary metabolism and a conco mitant drop in thiophene biosynthesis. The major part of the reduction was caused by a drop in enzyme activity, Substrate availability playe d a minor role. The results indicate that sulphur is involved in the m olecular control of secondary metabolism in Tagetes.