Downloaded from www.microbiologyresearch.org by IP: 34.229.49.225 On: Tue, 16 Jan 2018 21:32:33 Journal of General Microbiology (1986), 132, 3315-3323. Printed in Great Britain 3315 Glutamine Synthesis Regulates Sucrose Catabolism in Neurosporu crussu By GEORGINA HERNANDEZ AND JAIME MORA* Centro de Investigacih sobre Fijacicin de Nitrbgeno, Universidad Nacional Authornu de Mhxico, Cuernavaca, Mor., Mexico (Received 30 April 1986) The effect of glutamine synthesis on sucrose metabolism in Neurospora crassa was studied. Different inhibitors of glutamine synthetase were used to inhibit glutamine synthesis in mutants having a low ammonium assimilation capacity. Sucrose utilization was impaired, as indicated by a lower concentration and synthesis of intermediates of the tricarboxylic acid cycle and reduced release of COz. We propose that a coordinated regulation of carbon and nitrogen utilization is achieved through sensing of the carbon and nitrogen flows through glutamine synthesis, mediated by changes in the intracellular content of ATP, which is reduced as a consequence of glutamine synthesis. INTRODUCTION Carbon and nitrogen metabolism interact in the first instance in the reactions of ammonium assimilation. Carbon compounds, energy and reducing power are required for glutamate and glutamine synthesis. In Neurospora crassa glutamate can be synthesized through the participation of two different enzymes, glutamate dehydrogenase (GDH) (EC 1.4.1.4), NADPH dependent, which synthesizes glutamate from 2-oxoglutarate and ammonium (Fincham, 1950; Hernandez et al., 1983) and glutamate synthase (GOGAT) (EC 1.4.1.14), NADH dependent, which gives two molecules of glutamate from glutamine and 2-oxoglutarate (Hummelt & Mora, 1980a, b). Glutamine synthetase (GS) (EC 6.3.1.2) requires ATP and in N. crussa it is found in two different oligomeric forms composed of cc and p monomers respectively (Davila et al., 1980). Carbon and nitrogen metabolism also interact in the metabolic steps related to the distribution of nitrogen from glutamate and glutamine into several biosynthetic pathways. In N. crassa the nitrogen from glutamine is distributed by the enzymes of the o-amidase pathway as well as by transamidation. Glutamine transaminase synthesizes different amino acids and o- amidase hydrolyses the resulting 2-oxoglutaramate to 2-oxoglutarate and ammonium (Calderon et al., 1985).The ammonium released by the o-amidase pathway is assimilated not only by GDH but also by GS, thus leading to the operation of a glutamine cycle in which this amino acid is continually degraded and resynthesized (Calderbn et al., 1985 ; Calderon & Mora, 1985). GOGAT also participates in the distribution of the nitrogen from glutamine (Calderon & Mora, 1985). The regulation by the nitrogen source of the enzymes that synthesize glutamate or glutamine (Hernindez et al., 1983; Vichido et al., 1978; Quinto et al., 1977; Shnchez et al., 1978) and the participation of these enzymes in different ammonium assimilation pathways in N . crusm (Lara et al., 1982) have been reported. There have also been reports on the regulation of GS and of GDH by the carbon source in N. crassa and other micro-organisms (Ferguson & Sims, 1974a, b; Hemmings, 1978; Kapoor & Grover, 1970). Mora et al. (1980) reported that when mycelium of Abbreviations: GDH, glutamate dehydrogenase; GOGAT, glutamate synthase; GS, glutamine synthetase; MS, methionine sulphoximine. 0001-2993 0 1986 SGM