36 Integration of Multiple Regulatory Inputs in the Control of HO Expression in Yeast

Abstract

OVERVIEW
The HO gene of the yeast Saccharomyces cerevisiae is subject to multiple forms of regulation–expressed only in certain cell types, only in one phase of the cell cycle, and only in certain cells of a mitotic cell lineage. These different modes of regulation are individually of considerable interest, but it is perhaps of special interest to understand how they are coordinated with each other. This coordination takes place in an up-stream regulatory region of 1500 base pairs that is governed by at least eight SWI and SNF genes required for transcription of HO and at least five SIN genes, which mediate some type of negative regulation. Many of these regulatory proteins play roles in transcription of several other yeast genes, including G1 cyclins. Some are known to be components of chromatin; others identify a regulatory activity (“SWI1, 2, 3”) that appears to be present in other organisms, including Drosophila and vertebrates.

INTRODUCTION
The yeast HO gene is subject to three different modes of regulation that are coordinated with each other. The upstream region of HO can be viewed as a “regulatory ganglion” where multiple inputs are integrated to generate an output signal, transcription of the HO gene. There are three inputs, one concerned with monitoring cell type, the second with position in the cell cycle, and the third with cell lineage. HO is transcribed only when the three inputs are appropriate.

Many products required for transcription of HO have been identified, primarily on the basis of mutant isolation...