A CBF5 mutation that disrupts nucleolar localization of early tRNA biosynthesis in yeast also suppresses tRNA gene-mediated transcriptional silencing

In the budding yeast, Saccharomyces cerevisiae, actively transcribed tRNA g enes can negatively regulate adjacent RNA polymerase II (pol II)-transcribe d promoters. This tRNA gene-mediated silencing is independent of the orient ation of the tRNA gene and does not require direct, steric interference wit h the binding of either upstream pol II factors or the pol II holoenzyme. A mutant was isolated in which this form of silencing is suppressed. The res ponsible point mutation affects expression of the Cbf5 protein, a small nuc leolar ribonucleoprotein protein required for correct processing of rRNA. B ecause some early steps in the S. cerevisiae pre-tRNA biosynthetic pathway are nucleolar, we examined whether the CBF5 mutation might affect this loca lization. Nucleoli were slightly fragmented, and the pre-tRNAs went from th eir normal, mostly nucleolar location to being dispersed in the nucleoplasm . A possible mechanism for tRNA gene-mediated silencing is suggested in whi ch subnuclear localization of tRNA genes antagonizes transcription of nearb y genes by pol II.