Distribution of U3 small nucleolar RNA and fibrillarin during early embryogenesis in Caenorhabditis elegans.

U3 small nucleolar RNA (snoRNA) is one of the members of the box C/D class of snoRNA and is essential for ribosomal RNA (rRNA) processing to generate 18S rRNA in the nucleolus. Although U3 snoRNA is abundant, and is well conserved from yeast to mammals, the genes encoding U3 snoRNA in C. elegans have long remained unidentified. A recent RNomics study in C. elegans predicted five distinct U3 snoRNA genes. However, characterization of these candidates for U3 snoRNA has yet to be performed. In this study, we isolated and characterized four candidate RNAs for U3 snoRNA from the immunoprecipitated RNAs of C. elegans using an antibody against the 2,2,7-trimethylguanosine (TMG) cap. The sequences were identical to the predicted U3 sequences in the RNomics study. Here, we show the several lines of evidence that the isolated RNAs are the true U3 snoRNAs of C. elegans. Moreover, we report the novel expression pattern of U3 snoRNA and fibrillarin, which is an essential component of U3 small nucleolar ribonucleoprotein complex, during early embryo development of C. elegans. To our knowledge, this is the first observation of the inconsistent localization U3 snoRNA and fibrillarin during early embryogenesis, providing novel insight into the mechanisms of nucleologenesis and ribosome production during early embryogenesis.