Nucleolar clustering of dispersed tRNA genes

Abstract

Early transfer RNA (tRNA) processing events in Saccharomyces cerevisiae are co-ordinated in thenucleolus, the site normally associatedwith ribosomebiosynthesis. To test whether spatial organization of the tRNA pathway begins with nucleolar clustering of the genes, we have probed the subnuclear location of five different tRNA gene families. The results show that tRNA genes, though dispersed in the linear genome, colocalize with 5S ribosomal DNA and U14 small nucleolar RNA at the nucleolus. Nucleolar localization requires tRNA gene transcription-complex formation, because inactivation of the promoter at a single locus removes its nucleolar association. This organization of tRNA genes must profoundly affect the spatial packaging of the genome and raises the question of whether gene types might be coordinated in three dimensions to regulate transcription. Little is known about how specific nuclear genes are arranged in three dimensions, al-though the position of some chromosome regions are dynamic and responsive to th