CAENORHABDITIS-ELEGANS EMBRYOS CONTAIN ONLY ONE MAJOR SPECIES OF RO RNP

In virtually all vertebrate cells, Ro RNPs consist of the 60-kDa Ro au toantigen bound to one of several small cytoplasmic RNA molecules know n as Y RNAs. Because the 60-kDa Ro autoantigen is also found complexed with defective precursors of 5S rRNA in Xenopus oocytes, we have prop osed that this protein functions in a quality control, or discard path way, for 5S RNA biosynthesis (O'Brien CA, Wolin SL, 1994, Genes & Dev 8:2891-2903). The role of the Y RNAs in this pathway is unknown. To be gin a genetic analysis of Ro RNP function, we have characterized these particles in the nematode Caenorhabditis elegans. The C. elegans Ro p rotein is 12 kDa larger than the vertebrate protein; the larger size i s due in part to an N-terminal extension and to two insertions in the RNA recognition motif. In contrast to all previously described vertebr ate species, the Ro protein appears bound to a single Y RNA in C. eleg ans. Similar to vertebrate Y RNAs, the C. elegans Y RNA can be folded to form a pyrimidine-rich internal loop and a long stem in which the 5 ' and 3' ends are base paired. Within the stem is a conserved bulged h elix that is proposed to be the binding site of the Ro protein. Intere stingly, although the human protein can bind the nematode Y RNA, the C . elegans protein does not bind human Y RNAs. This is the first descri ption of Ro RNPs in an invertebrate species.