SYNTHESIS OF FUNCTIONAL EUKARYOTIC RIBOSOMAL-RNAS IN TRANS - DEVELOPMENT OF A NOVEL IN-VIVO RDNA SYSTEM FOR DISSECTING RIBOSOME BIOGENESIS

Active 18S and 25S ribosomal RNAs were produced in trans in yeast, fro m plasmids containing RNA polymerasc II transcription signals and rDNA fragments with unique hybridization tags. Analyses were carried out i n cells with temperature-sensitive RNA polymerase I. Functional rRNAs were derived from separate 18S and 5.8/25S rRNA coding units, however, active 25S rRNA could be produced only by cotranscription with 5.8S r RNA. The results demonstrate that the polycistronic organization of th e large rDNA operon is not required for successful processing of I-RN, I or assembly of functional ribosomes, The split operon system should facilitate future efforts to dissect eukaryotic ribosome biogenesis.