A NOVEL SNORNA (U73) IS ENCODED WITHIN THE INTRONS OF THE HUMAN AND MOUSE RIBOSOMAL-PROTEIN S3A GENES

The mouse ribosomal protein S3a-encoding gene (mRPS3a) was cloned and sequenced in this study, mRPS3a shares identical exon/intron structure with its human counterpart. Both genes are split to six exons and exh ibit remarkable conservation of the promoter region (68.8% identity in the 250 bp upstream of cap site) and coding region (the proteins diff er in two amino acids). mRPS3a displays many features common to other r-protein genes, including the CpG-island at 5'-end of the gene, cap s ite within an oligopyrimidine tract and no consensus TATA or CAAT boxe s. However, mRPS3a represents a rare subclass of r-protein genes that possess a long coding sequence in the first exon. Comparison of human and mouse S3a genes revealed sequence fragments with striking similari ty within introns 3 and 4. Here we demonstrate that these sequences en code for a novel small nucleolar RNA (snoRNA) designated U73. U73 cont ains C, D and D' boxes and a 12-nucleotide antisense complementarity t o the 28S ribosomal RNA. These features place U73 into the family of i ntron-encoded antisense snoRNAs that guide site-specific 2'-O-ribose m ethylation of pre-rRNA. We propose that U73 is involved in methylation of the G1739 residue of the human 28S rRNA. In addition, we present t he mapping of human ribosomal protein S3a gene (hRPS3a) and internally nested U73 gene to the human chromosome 4q31.2-3. (C) 1998 Elsevier S cience B.V.