Nitrogen Metabolism in Saccharomyces cerevisiae

INTRODUCTION In decaying fruit and vegetable matter, which form their wild habitat, yeast cells encounter a broad spectrum of compounds able to serve as nitrogen sources. These materials range from simple ammonia and amino acids to complex nucleic acids and their derivatives. In response to this heterogeneous environment, Saccharomyces cerevisiae has evolved equally broad degradative enzyme systems and sophisticated ways of regulating and integrating their operation.1 In the past, enteric bacteria have served as a model for viewing the control of nitrogen metabolism in yeast. As a starting point, these past paradigms have served us well. However, they are certain to be inadequate for the future. An important area of deficiency is their failure to involve the complex internal structure of a yeast cell in the control and integration of nitrogen metabolism. Ignoring such structure and its potential role in cellular homeostasis may mortgage our understanding of a yeast cell’s ability to cope with a constantly changing environment. A SUMMARY OF THE MAJOR CATABOLIC SYSTEMS As shown in Table 1, S. cerevisiae will grow fairly well on many amino acids, uracil, purine derivatives, urea, and ammonia. Sometimes the cells go through only one or two generations (this is indicated in Table 1) before growth ceases. In these cases, the conclusion that cells are able to use the compound effectively must be viewed skeptically. However, in other cases, steady-state growth can easily be maintained. Only a limited number of these compounds and their metabolic fates have been studied; they are briefly...