Nitrogen source regulates the biosynthesis of NADP-glutamate dehydrogenase in the ectomycorrhizal basidiomycete Laccaria bicolor

NADP-glutamate dehydrogenase (NADP-GDH) is a key enzyme of N assimilation in ectomycorrhizal fungi, but little is known about its regulation. In the present study, the effects of N source and concentration on NADP-GDH specific activity, polypeptide content and biosynthesis have been investigated in the ectomycorrhizal basidiomycete Laccaria bicolor S238N. Mycelium was transferred from NH4+-rich medium to either NO3--rich or N-free medium. Under both these conditions, intracellular NH4+ and glutamine content decreased dramatically. The specific activity of NADP-GDH was simultaneously increased 3-4.5-fold. Immunological analysis using anti-GDH antibodies showed that depression of enzyme activity resulted from increased concentration of the GDH polypeptide. In addition, assessment of the steady-state biosynthesis of NADP-GDH by in vivo labelling of proteins demonstrated that the observed changes in enzyme concentration are explained by differences in the relative rate of enzyme biosynthesis. These results suggested that the source of N regulates synthesis of NADP-GDH and that this regulation is under translational and/or transcriptional control.