The hypothesis is advanced that NADP+-malic enzyme (ME; EC 1.1 .1.40) is an important activity in regulating the extent of lipid accumulation in filamentous fungi. In Mucor circinelloides, a fungus capable of accumulating only 25% (w/w, dry wt) lipid, even under the most propitious conditions, M E disappears 15-20 h after nitrogen exhaustion, coincident with the cessation of lipid accumulation. In contrast, M E in Mortierella alpina, a fungus capable of accumulating 50% (w/w, dry wt) lipid, remains active for over 60 h after Nexhaustion during which time lipid accumulation continues. No other enzyme activity studied, including the lipogenic enzymes acetyl-CoA carboxylase, fatty acid synthase, diacyglycerol acyltransferase, ATP : citrate lyase and the NADPHgenerating enzymes g lucose-6-phosphate de h ydrogenase, 6-p hosp hog I uconate dehydrogenase and NADP+ : isocitrate dehydrogenase, demonstrated any correlation with the accumulation of storage lipid in either fungus. Full activity of M E is restored in Mr. circinelloides within 4 h by adding NH; to the cultures, but this is prevented by adding cycloheximide as an inhibitor of protein synthesis. This suggests that the decrease in M E activity occurs due to down-regulation of the M E gene.I

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