Constitutive and Coordinately Regulated Transcription of Yeast Genes: Promoter Elements, Positive and Negative Regulatory Sites, and DNA Binding Proteins

The yeast genome specifies approximately 5000 protein coding genes that are densely clustered on 16 linear chromosomes. In the form of nuclear chromatin, these genes are transcribed by RNA polymerase II from discrete initiation sites. The average yeast gene is transcribed about five to ten times during each cell cycle which results in a steady-state mRNA level of one to two molecules per cell. However, some genes are transcribed constitutively at considerably different rates, whereas other genes are transcribed at variable rates depending on the physiological circumstances. Such regulated expression is achieved either by positive factors that increase transcription above a basal level or by negative factors that repress transcription below a basal level (for a general review of yeast promoters, see Struhl 1985a).