Electron microscopy of the Arthus reaction, using ferritin as antigen.

I. INTRODUCTION Studies of yeast cell types and their interconversion have provided fertile ground for elucidating molecular mechanisms of gene regulation, recombination, and signal transduction. This chapter focuses on gene regulation and recombination; signal transduction is discussed in detail by Sprague and Thorner (this volume) and elsewhere (Marsh et al. 1991). For an introduction to mating type, see Herskowitz (1988a). A comparison of this review with its predecessor (Herskowitz and Oshima 1981) will reveal a remarkable amount of progress. Yeast has two haploid mating types, a and α, that mate to form the third cell type, the a /α cell. In Section II, we discuss how cell type is regulated by proteins encoded by the mating-type locus. These regulatory proteins, some of which (such as homeodomain proteins Matα2p and Mat a 1p) are closely related to those of multicellular eukaryotes, govern transcription of gene sets whose members have roles in signal transduction and other processes. These studies reveal that cell specialization results from the combined action of regulatory proteins and environmental signals (pheromones from mating partners or nutritional starvation). The regulatory proteins themselves in many cases are combinations of different monomeric subunits that associate to generate specific regulatory proteins. Studies of some of these regulatory proteins, notably Matα2p, for which a crystal structure of Matα2p bound to its operator has been determined, have reached a high level of molecular sophistication. Yeast cells of one mating type can switch to the other mating type in a process that is a programmed genetic...