Transposable element sequences involved in the enhancement of yeast gene expression

The his4-917 mutation of yeast results from the insertion of a Ty element, Ty917, into the 5' regulatory region of the HIS4 gene. Ty917 prevents HIS4 transcription, thus rendering the cell histidine requiring. Recombination between Ty917 and a Ty element elsewhere in the yeast genome can result in the replacement of part or all of the Ty917 element by sequences from the Ty element. Recombinant derivatives display a variety of phenotypes including His-, weakly His+, and strongly His+. In most of the His+ derivatives, the expression of HIS4 is controlled by genes at the mating type locus. To identify the Ty sequences important in controlling the expression of an adjacent gene, we used Ty elements that have different effects on gene expression to construct hybrid Ty elements in vitro. The effects of these hybrid elements on HIS4 expression were examined. These experiments indicate that the critical sequence differences between Ty elements that permit HIS4 expression and those that prevent its expression lie in the rightmost (HIS4-proximal) 730 base pairs of the element. The DNA sequence of this region was determined for three elements: Ty917, which prevents HIS4 expression; Ty917(467), which confers a weak His+ phenotype; and Ty917(480), which confers a strong His+ phenotype. Within this region, Ty917(467) differs from Ty917 by a single base-pair change that is in the internal (epsilon) region of the Ty element. Ty917(480) differs from Ty917 by this same base-pair change and by 10 changes in the terminal delta sequence. The sequence change common to Ty917(467) and Ty917(480) lies in a region of the Ty element that is homologous to the simian virus 40 enhancer of transcription.