Uridine Diphosphoglucose Pyrophosphorylase Activity and Differentiation in the Acellular Slime Mold Physarum polycephalum

Abstract

The specific activity of uridine 5′-triphosphate:α- d -glucose 1-phosphate uridyltransferase (EC 2.7.7.9) (also called uridine 5′-diphosphate [UDP]-glucose pyrophosphorylase) has been found to increase up to eightfold during spherule formation by the slime mold Physarum polycephalum . The enzyme accumulates during the first 8 to 9 h after initiation of spherule formation, declines to basal levels found in vegetative microplasmodia by 15 h, and is undetectable in completed spherules. Specific activities for UDP-glucose pyrophosphorylase in vegetative microplasmodia range from 15 to 30 nmol of UDP-glucose formed per min per mg of protein, whereas accumulated levels during spherule formation can attain a specific activity as high as 125 nmol of UDP-glucose formed per min per mg of protein. The scheduling and extent of accumulation are critically dependent on an early log-phase age of microplasmodia originally induced to form spherules. Spherule induction by 0.2 M or 0.5 M mannitol delays this schedule in a variable and unpredictable manner. Spherule-forming microplasmodia which have accumulated high levels of UDP-glucose pyrophosphorylase spontaneously excrete the enzyme when transferred to salts medium containing 0.2 M or 0.5 M mannitol. The excreted enzyme is subsequently destroyed or inactivated. Studies with preferential inhibitors of macromolecular synthesis indicate that accumulation of UDP-glucose pyrophosphorylase requires concomitant protein synthesis and prior ribonucleic acid synthesis.