RESPONSE OF AROGENATE DEHYDRATASE TO CHANGING PHYSIOLOGICAL STATES OF GROWTH IN SUSPENSION-CULTURES OF NICOTIANA-SILVESTRIS

The well-documented differential expression of isozymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase, DS-Mn (plastid) and DS-Co (cytosol), provide a basis for comparing the expression of other aromatic-pathway enzymes in Nicotiana silvestris. Arogenate dehydratase, catalyzing the final step of L-phenylalanine biosynthesis in the plastid, is expressed at its highest level during the exponential phase of growth. Maximal activity is achieved at mid-exponential growth, and activity declines with the onset of stationary-phase physiology. Thus, DS-Mn and arogenate dehydratase show similar behavior with respect to the overall relationship of activity level and physiological state of growth. However, the detailed kinetics of enzymic response to the subculture of stationary-phase cells in suspension culture differed for DS-Mn and arogenate dehydratase. Arogenate dehydratase exhibited a later onset time of activity increase and the duration of activity increase was sustained longer than was the case for DS-Mn.