The Regulation of Nitrate Assimilation in Yeast

Abstract

Since the earlier investigations of workers such as Silver (1957) and Pichinoty and Metenier (1967), progress in the study of the structure and regulation of the enzymes of nitrate assimilation in yeasts has been disappointing, compared with filamentous fungi and unicellular algae. However, more recent investigations with the imperfect, asporogenous yeast, Candida nitratophila, have yielded much useful information and this has helped to fill the gaps in our knowledge (Hipkin 1989). It is clear from all studies that the nitrate assimilation pathway in yeasts is similar to that found in other organisms that use nitrate as a nitrogen source and our current understanding of the pathway that incorporates nitrate-nitrogen into a-amino nitrogen in C. nitratophila is outlined in Figure 1. There are several noteworthy features here. Firstly, in yeasts, the heterotrophic reduction of nitrate to ammonium (via nitrite) can be driven entirely by reducing power in the form of NADPH. In species like C. nitratophila, the enzyme nitrate reductase (NR) is able to use NADH or NADPH as an electron donor whereas nitrite reductase is NADPH-specific (Hipkin 1989). Secondly, although it has been accepted widely that the NADPH-glutamate dehydrogenase pathway is the major pathway for the incorporation of ammonium nitrogen into a-amino nitrogen in nitrateutilizing yeasts (Sims and Folkes 1964; Brown et al. 1973; Miflin and Lea 1980), it is clear now that a significant amount of ammonium may be assimilated via the glutamate syn-thase cycle in these organisms under non-nitrogen-replete conditions; e.g. when nitrate is the sole source of nitrogen being assimilated (Hipkin et al. 1990). Thirdly, although many forms of reduced nitrogen could be regarded as end products of nitrate assimilation, ammonium, glutamate and glutamine are the ones that occupy a central position in terms of the regulation of nitrate metabolism.