GAR2 IS A NUCLEOLAR PROTEIN FROM SCHIZOSACCHAROMYCES-POMBE REQUIRED FOR 18S RIBOSOMAL-RNA AND 40S RIBOSOMAL-SUBUNIT ACCUMULATION

Several nucleolar proteins, such as nucleolin, NOP1/fibrillarin, SSB1, NSR1 and GAR1 share a common glycine and arginine rich structural mot if called the GAR domain. To identify novel nucleolar proteins from fi ssion yeast we screened Schizosaccharomyces pombe genomic DNA librarie s with a probe encompassing the GAR structural motif. Here we report t he identification and characterization of a S.pombe gene coding for a novel nucleolar protein, designated gar2. The structure of the fission yeast gar2 is reminiscent of that of nucleolin from vertebrates and N SR1 from Saccharomyces cerevisiae. In addition, like these proteins, g ar2 has a nucleolar localisation. The disruption of the gar2+ gene aff ects normal cell growth, leads to an accumulation of 35S pre-rRNA and a decrease of mature 18S rRNA steady state levels, Moreover, ribosomal profiles of the mutant show an increase of free 60S ribosomal subunit s and an absence of free 40S ribosomal subunits, gar2 is able to rescu e a S.cerevisiae mutant lacking NSR1, thus establishing gar2 as a func tional homolog of NSR1. We propose that gar2 helps the assembly of pre -ribosomal particles containing 18S rRNA.