THE YEAST NOP4 GENE-PRODUCT IS AN ESSENTIAL NUCLEOLAR PROTEIN REQUIRED FOR PRE-RIBOSOMAL-RNA PROCESSING AND ACCUMULATION OF 60S RIBOSOMAL-SUBUNITS

Saccharomyces cerevisiae NOP4 gene was isolated by screening a lambda gt11 yeast genomic DNA library with a monoclonal antibody against a ye ast nucleolar protein. NOP4 encodes a 78 kDa protein that contains two prototypical RNA recognition motifs (RRMs) flanking an imperfect RRM lacking characteristic RNP1 and RNP2 motifs. In addition, there is a f ourth incomplete RRM. NOP4 is a single copy essential gene present on chromosome XVI, between RAD1 and PEP4. To examine the function of Nop4 p, we constructed a conditional null allele of NOP4 by placing this ge ne under the control of the glucose-repressible GAL1 promoter. When ce lls are shifted from galactose-containing medium to glucose-containing medium, NOP4 transcription is terminated, Nop4 protein is depleted an d cell growth is impaired. Nop4 protein depletion results in diminishe d accumulation of 60S ribosomal subunits, assignable to a defect in ri bosome biogenesis arising from a lack of production of mature 25S rRNA from 27S precursor rRNA.