DOES SACCHAROMYCES NEED AN ORGANIZED NUCLEOLUS

The ribosomal RNA (rRNA) genes of most eukaryotic organisms are arrang ed in one or more tandem arrays, often termed nucleolar organizer regi ons. The biological implications of this tandem organization are not k nown. We have tested the requirement for such a chromosomal organizati on by directly comparing the transcription and processing of rRNA in i sogenic strains of Saccharomyces cerevisiae that differ only in the or ganization of their rRNA genes. Strain L-1489 carries the RDN locus, c onsisting of 100-150 copies of the rRNA genes in a tandem array on chr omosome XII. Strain L-1521 has a complete deletion of the RDN array, b ut carries many copies of a plasmid that includes a single rRNA gene. While this strain grows reasonably well, the average transcriptional a ctivity of the plasmid genes is substantially less than that of the ch romosomal copies. However, there is little difference in the processin g of the 35S pre-rRNA to the mature 25S:5.8S and 18S products. Thus, n either a chromosomal location nor a tandem repeat of the rRNA genes is important for the assembly and function of the many protein and RNA m olecules necessary for the processing of the rRNA transcripts. We inve stigated the consequence of a dispersed gene arrangement on nucleolar structure. Immunofluorescence microscopy revealed that in strain L-152 1 the yeast fibrillarin, Nop1p, rather than being confined to a define d nucleolus at the edge of the nucleus as it is in cells with the norm al arrangement of rRNA genes, is spread throughout the nucleus. This o bservation implies that each plasmid rRNA gene can serve as a nucleola r organizer. Finally, data from pulse-labeling experiments show that t he repression of rRNA transcription due to failure of the secretory pa thway is independent of whether the rRNA genes are at the RDN locus on chromosome XII or on plasmids. This result suggests that the regulati on of rRNA transcription occurs at the level of soluble factors rather than chromatin structure.