NITRATE REDUCTASE EXPRESSION IN MAIZE LEAVES (ZEA-MAYS) DURING DARK-LIGHT TRANSITIONS - COMPLEX EFFECTS OF PROTEIN PHOSPHATASE INHIBITORS ON ENZYME-ACTIVITY, PROTEIN-SYNTHESIS AND TRANSCRIPT LEVELS

The effects of cytoplasmic protein synthesis and protein phosphatase a ctivity on NADH:nitrate reductase (NR) activity, protein and transcrip t were examined in maize (Zea mays L.) seedling leaves. A rapid increa se in NR activity, measured in the presence of 5 mM Mg2+, was found up on exposure of excised leaves to light. Inhibitors of protein phosphat ase activity (okadaic acid [OKA] and microcystin [MC]-LR) completely p revented the increase in NR activity. The cytoplasmic protein synthesi s inhibitor, cycloheximide (CHX), did not affect Mg2+ inhibition of NR activity during the dark-to-light transition. V-max NR activity, meas ured in the presence of P-i and EDTA, remained constant or increased s lightly in maize leaves during the first 2 h of the light period. OKA, MC-LR or CHX treatment caused a 40 to 50% reduction in V-max NR activ ity during this time. Incorporation of S-35-Met into NR protein was re duced more than 90% by CHX and 80% by OKA. The inhibition of NR protei n synthesis by CHX and OKA correlated with a 50 to 60% decrease in S-3 5-Met incorporation into total soluble protein over the treatment peri od. The increase in NR mRNA levels early in the light period was preve nted by OKA and MC-LR, but not by CHX. OKA had a similar effect on suc rose phosphate synthase mRNA levels, but did not affect Catalase 1 or Catalase3 mRNA accumulation. The data suggest that light-induced decre ases in Mg2+ inhibition of NR activity and transcript levels are indep endent of new protein synthesis. The effects of OKA and MC-LR indicate that protein phosphatase activities could be involved, directly or in directly, in the regulation of NR activity, protein synthesis and tran script accumulation.