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Lipid Research Group, Department of Biological Sciences, University of Hull, Hull HU6 7RX, UK
Present address: Department of Microbiology, University of Kebangsaan Malaysia, 43 600 Bangi, Salangor Darul Ehsan, Malaysia.
Author for correspondence: Colin Ratledge. Tel: + 44 1482 465243. Fax: + 44 1482 465458 e-mail: c.ratledge@biosci.hull.ac.uk
ABSTRACT
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, ME disappears 15-20 h after nitrogen exhaustion, coincident with the cessation of lipid accumulation. In contrast, ME in Mortierella alpina, a fungus capable of accumulating 50% (w/w, dry wt) lipid, remains active for over 60 h after N-exhaustion 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 NADPH-generating enzymes glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and NADP+: isocitrate dehydrogenase, demonstrated any correlation with the accumulation of storage lipid in either fungus. Full activity of ME is restored in Mr. circinelloides within 4 h by adding NH+4 to the cultures, but this is prevented by adding cycloheximide as an inhibitor of protein synthesis. This suggests that the decrease in ME activity occurs due to down-regulation of the ME gene.
Abbreviations: ACC, acetyl-CoA carboxylase; ACL, ATP:citrate lyase; DAGAT, diacylglycerol acyltransferase; FAS, fatty acid synthase; G-6-PDH, glucose-6-phosphate dehydrogenase; ICDH, isocitrate dehydrogenase; ME, malic enzyme; 6-PGDH, 6-phosphogluconate dehydrogenase.
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