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Abstract
Ty elements are a family of dispersed repeated transposable elements found in the genome of Saccharomyces cerevisiae. Many mutations have been isolated in which a Ty element is integrated upstream of a gene and assumes regulatory control over the expression of the adjacent gene. The purpose of the experiments presented in this thesis is to identify cis-acting sequences within a Ty element important for adjacent gene expression. The importance of these sequences to Ty transcription is also analyzed in an attempt to establish the relationship between Ty and adjacent gene expression. Three naturally occurring Ty elements located at the HIS4 locus were examined. These Ty elements differ by multiple sequence changes and have different effects on HIS4 expression. To determine which sequence changes are important to Ty and HIS4 expression, hybrid Ty elements were generated and these were used to construct Ty:lacZ and Ty:HIS4:lacZ fusion genes. These experiments indicate the presence of a sequence element in the Ty epsilon region which is necessary for HIS4 expression but has only a modest effect on Ty transcription. This sequence element may function as an enhancer or upstream activator of adjacent gene transcription. In addition, a point mutation in the Ty promoter region decreases Ty transcription and increases HIS4 expression.
The opposite effects of the Ty promoter mutation on Ty and adjacent gene expression are probably due to promoter competition; decreased transcription from the HIS4 promoter in the Ty $\delta$. This possibility was investigated by deletion of the Ty promoter; this deletion results in a further increase of HIS4 expression. Ty elements containing the "wild-type" promoter, the point mutation and the deletion were placed upstream of a CYC1 gene lacking the upstream activation sequence necessary for wild-type CYC1 transcription. The mutations in the Ty promoter caused an even greater increase of CYC1 expression than of HIS4 expression indicating that promoter competition between the Ty and the adjacent CYC1 promoter is more pronounced.
