6 Regulation of Nitrogen Utilization

Abstract

I. INTRODUCTION
This chapter deals with the changes in levels of enzymes and permeases that occur in the yeast cell in response to alterations in the availability of nitrogen sources in the growth medium. In some cases, the presence of a particular nitrogen compound in the growth medium induces the formation of the enzymes required for its utilization. Furthermore, it has long been known that the presence of nitrogen sources that support optimal growth, such as glutamine, asparagine, and ammonia, decreases the level of enzymes required for the utilization of poorer nitrogen sources, such as arginine, proline, and urea; this phenomenon has been called nitrogen catabolite repression (Cooper 1982a; Wiame et al. 1985). I consider this name, which seems to imply that the catabolism of the repressing nitrogen compounds plays a role in the regulation, to be incorrect and patently absurd when applied to ammonia. I therefore refer to the phenomenon that is the subject of this chapter by the neutral term nitrogen regulation.

The proteins largely responsible for nitrogen regulation are the products of three genes, GLN3, URE2, and GAP1. Gln3p is the activator of the expression of many genes whose products are responsible for the utilization of diverse sources of nitrogen, including GAP1, the structural gene for the low-affinity, high-capacity, general amino acid permease, GAP. This permease facilitates the entry into the cell of amino acids capable of inducing the enzymes required for their own degradation and thus for their utilization as general sources of nitrogen. Ure2p disables...