THE NOP60B GENE OF DROSOPHILA ENCODES AN ESSENTIAL NUCLEOLAR PROTEIN THAT FUNCTIONS IN YEAST

The Cbf5 protein of Saccharomyces cerevisiae was originally identified as a low-affinity centromeric DNA-binding protein, and cbf5 mutants h ave a defect in rRNA synthesis. A closely related protein from mammals , NAP57, is a nucleolar protein that coimmunoprecipitates with the nuc leolar phosphoprotein Nopp 140. To study the function of this protein family in a higher eukaryote that is amenable to genetic approaches, t he gene encoding a Drosophila melanogaster homolog, Nop60B, was identi fied. The predicted Drosophila protein shares a high degree of sequenc e identity over a 380-residue region with both the mammalian and yeast proteins, and shares several conserved motifs with the prokaryotic tR NA pseudouridine 55 synthases. Nop60B RNA is found at high levels in n urse cells and in the oocyte, and is present throughout development. N op60B protein is localized primarily to the nucleolus of interphase ce lls? and is absent from the chromosomes during mitosis. Nop60B mutants were generated and shown to be homozygous lethal. The Drosophila gene can rescue the lethal phenotype of yeast cbf5 mutations, showing that the function of this protein has been conserved from yeast to Drosoph ila.