PLASTID-LOCALIZED 3-DEOXY-D-ARABINO-HEPTULOSONATE 7-PHOSPHATE SYNTHASE (DS-MN) - THE EARLY-PATHWAY TARGET OF SEQUENTIAL FEEDBACK INHIBITIONIN HIGHER-PLANTS

A plastid-localized isozyme of 3-deoxy-D-arabino-heptulosonate 7-phosp hate (DAHP) synthase, denoted DS-Mn, has been identified in a number o f higher-plant species. Parallel characterizations were made of DS-Mn from Spinacia oleracea leaf tissue, Solanum tuberosum tubers, and Nico tiana silvestris suspension culture as sources of enzyme from plant ma terials which vary in phylogeny, developmental and tissue state, and p hysiological state. A highly conserved property of DS-Mn is a transiti on between inactive and active states, mediated by DTT as a hysteretic activator. A procedure for isolation of DS-Mn in the labile, inactive state is given. The process of activation appears to exhibit a higher pH optimum than the catalytic optimum. DTT-containing preparations ar e very stable. The enzyme characteristically exhibits stimulation by M n++ in the range of 45-50%, relatively high affinity for erythrose-4-p hosphate (E4P), dramatic substrate inhibition above about 0.5 mol m-3 E4P, sigmoid substrate saturation curves for both E4P and phosphoenolp yruvate, and inhibition by L-arogenate (competitive against E4P and no n-competitive against PEP). DS-Mn has a relatively high temperature op timum in the range of 45-50-degrees-C. Enzyme activity was lost when h ound metal was stripped away by EDTA treatment. Reconstitution of the native-enzyme level of activity was obtained with Ca++, and additional stimulation was achieved with Mn++. DS-Mn control by L-arogenate in t he chloroplast is proposed as one key circuit in an overall pattern of allosteric control for the entire pathway of aromatic amino acid bios ynthesis. This pattern is called sequential feedback inhibition. The p otential for modulation of this control system by environmental cues i nduced by light-dark transitions is discussed.