Publisher Summary
This chapter focuses on ammonia assimilation and discusses the mechanisms by which the ammonia produced is incorporated into organic compounds. Nitrate and dinitrogen gas, two of the major sources of inorganic nitrogen available to plants, are reduced to ammonia before the nitrogen is incorporated into organic matter. Ammonia is released and reassimilated in large amounts at different stages of the plant's metabolism. Nitrate is the major source of inorganic N available to the leaves of most land plants; ammonia is not present in the xylem stream leaving the roots. Thus, the ammonia assimilated is normally generated in situ from the reduction of nitrate through nitrite. Ammonia is not normally transported into a leaf but is rather a product of nitrate reduction; however, it can also be generated within the leaf by two other ways: (1) by the breakdown of asparagine through transaminase or asparaginase and (2) in photorespiration.

4 – Ammonia Assimilation

Introduction L-Amino acids General Alanine, aspartate and asparagine Arginine, proline and ornithine Glutamate and glutamine Branched chain amino acids Lysine Methionine and methionine sulphoxide Aromatic and heterocyclic amino acids Serine and threonine D-Amino acids o-Amino acids Purines Amides Primary, secondary and tertiary amines and choline Primary amines Secondary and tertiary amines Trimethylamine N-oxide Choline and ethanolamine Diand polyamines Pyrimidines The role of subcellular organelles in the breakdown of organic nitrogen compounds Mobilization of endogenous nitrogen reserves The use of organic nitrogen compounds as carbon sources Conclusions and perspectives Acknowledgements References

Degradation of organic nitrogen compounds by yeasts

Publisher Summary The chapter focuses on the physiology and metabolic controls of fungal growth The chapter considers those areas of fungal physiology and biochemistry that have been researched in depth, and attempts to indicate areas that are unexplored In addition, it explains the modulation of fungal metabolism in response to growth conditions Palpable absences from this chapter are references to “secondary” or “shunt” metabolism, at which fungi are most adept, and to the metabolic control of differentiation: in both of these areas, notable advances have been made in recent years The chapter also considers some aspects of macromolecule synthesis, particularly the synthesis of RNA and protein, in terms of overall rate-limiting processes in fungal growth Strain instability also has plagued the efforts of the fungal physiologist, and this has been an acute problem in relation to the Fungi Imperfecti many of which are of major interest in the context of commercial fermentations

The physiology and metabolic control of fungal growth.

NADH-dependent glutamate synthase and nitrogen metabolism in Neurospora crassa.

Inhibition of homocysteine sulfonamide of glutamate synthase purified from Saccharomyces cerevisiae.

The assimilation of ammonia in many yeasts is mediated by NADP-linked glutamate dehydrogenase. The synthesis of this enzyme appears to be derepressed during growth in environments containing low concentrations of ammonia or, as in Candida utilis, where nitrate is the nitrogen source (Brown & Johnson, 1970; Burn, Turner & Brown, 1974). An alternative pathway, common in bacteria (Tempest, Meers & Brown, I 973) , involves the enzymes glutamine synthetase and glutamate synthase. Glutamine synthetase is widespread in yeasts, while glutamate synthase has been detected in certain Schizosaccharomyces spp. (Brown, Burn & Johnson, 1973). Roon, Even & Larimore (1974) have reported the presence of glutamate synthase in some strains of Saccharomyces cerevisiae, together with much higher levels of both NADand NADP-linked glutamate dehydrogenase. We report the results of chemostat studies on the levels of enzymes involved in ammonia assimilation in representatives of the four recognized (Lodder, 1970) species of the genus Schizosaccharomyces and the single recognized species of the related genus Saccharomy codes.

The Enzymes of Ammonia Assimilation in Schizosaccharomyces spp. and in Saccharomycodes ludwigii

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The Enzymes of Ammonia Assimilation in Schizosaccharomyces spp. and in Saccharomycodes ludwigii