37 A Combinatorial Regulatory Circuit in Budding Yeast

Abstract

OVERVIEW
In budding yeast, three negative regulators of transcription (α2, a1, and MCM1) participate in combinatorial control circuits that specify cell type. Studies of these regulatory proteins have revealed the features that account for their abilities to act in various combinations. These studies have also provided insights into the way these regulatory proteins direct transcriptional repression.

INTRODUCTION
Different cell types of an organism contain different sets of proteins that regulate transcription. Cell-type-specific genes, in turn, are generally designed to respond to particular combinations of these regulatory proteins. These two ideas are commonly summarized by the term combinatorial control, instances of which have been documented in a wide variety of organisms (see Alberts et al. 1989).

Studies of cell-type specification in the yeast Saccharomyces cerevisiae have revealed a small-scale regulatory network of this type and have provided a number of insights into the design and behavior of the regulatory proteins that comprise the network. S. cerevisiae has three distinct types of cells termed a, α, and a/α. The a and α cells are normally haploid in chromosome number and are specialized for mating. An a cell mates with an α cell in a process that involves cell fusion followed by nuclear fusion and culminates in the formation of the third cell type, the a/α cell. Because it arises from the fusion of two haploid cells, the a/α cell is normally diploid in chromosome number. The a/α cell cannot mate; however, unlike the haploid cells types, it can be induced by the...