Inside the Inside: Part II: The Regulators. Cell Specialization: a Regulatory Hierarchy

Abstract

In outward appearance, the yeast cell is not exciting. One cell looks much like another, and each will give rise to clones having the same biochemical reactions. Nevertheless, appearances are deceiving, and each cell in its time plays many parts. There are, in fact, three cell types in yeast, each with a subtype, so that from some points of view there can be said to be six types in actual fact (Table 11.1). These are, as main types, two haploid cell types, a and α, and one diploid type, a/α, which can be considered a derivative of the other two, since it arises as the product of a mating between them. a cell types can mate with α types, after exposure to α mating hormone (subtype 1); α cell types can mate with a types, after exposure to a mating hormone (subtype 2), and a/α cells will not mate, are insensitive to both mating hormones, and have the potential to sporulate. a/α cells which are induced by starvation will sporulate, and the induced a/α cell constitutes the third subtype. The story of the control system by which all these activities are regulated by the cell is absorbing one, and shows the remarkable complexity of the mechanisms of the hierarchy of DNA binding proteins which govern the reproductive activities of what appears to be a simple yeast cell.