TRANSFER-RNA GENES AS TRANSCRIPTIONAL REPRESSOR ELEMENTS

Eukaryotic genomes frequently contain large numbers of repetitive RNA polymerase III (pol III) promoter element intrspersed between and within RNA pol II transcription units, and in several instances a regulatory relationship between the two types of promoter has been postulated. In the budding yeast Saccharomyces cerevisiae, tRNA genes are the only known interspersed pol III promoters in vivo. This inhibition requires active transcription of the upstream tRNA gene but is independent of its orientation and appears not to involve simple steric blockage of the pol II upstream activator sites. Evidence is presented that different pol II promoters can be repressed by different tRNA genes placed upstream at varied distances in both orientations. To test whether this phenomenon functions in naturally occurring instances in hich tRNA genes and pol II promoters are juxtapoed, we examined the sigma and Ty3 elements. This class of retrotransposons is always found integratd immediately upstream of different tRNA genes. Weakening tRNA gene transcription by means of a temperature-sensitive mutation in RNA pol II increases the pheromone-inducible expression of sigma and Ty3 elements up to 60-fold.