Enhancer-like stimulation of yeast tRNA gene expression by a defined region of the Ty element micro-injected into Xenopus oocytes.

In the yeast Saccharomyces cerevisiae, the majority of the tRNA genes are found associated with transposable elements one of which is the Ty element. We noticed that the transcriptional activity of several of these genes was rather different depending on the type of the accompanying element(s) [Nelböck et al. (1985) Biol. Chem. Hoppe-Seyler 366, 1041-1051]. In order to study the influence of a Ty element on tRNA gene expression, we used the method of micro-injection into Xenopus oocytes taking advantage of a Ty+/Ty- allelic pair of a tRNALys1 gene recently isolated in our laboratory. A direct comparison showed that the expression of the plus allele was c. sixfold higher than that of the minus allele. In order to determine sequences of the Ty responsible for this effect, we constructed a number of variants in which distinct segments of the Ty were deleted or rearranged. The activating ability was localized to a 590-bp segment of the Ty containing the 'promoter delta', independent of its orientation or location towards the tRNA gene thus resembling the effects described for transcriptional enhancers. This is the first time that a long-range effect has been observed on the expression of a gene transcribed by RNA polymerase III.