ABSTRACT
Small nucleolar RNAs (snoRNAs) constitute a group of more than 100 different stable RNA molecules that are found concentrated in the nucleolus where they are involved in the maturation of ribosomal RNA. Most snoRNAs are not produced from their own genes but are encoded in the introns of other genes, referred to as snoRNA host genes. Little is known about the mechanisms by which the snoRNAs are produced from introns and how the snoRNAs mature to functional snoRNP complexes in the nucleolus. One class of intron-encoded snoRNAs binds with high specificity to the protein fibrillarin which is found concentrated in the nucleolus, but also in small nuclear domains known as coiled bodies. It has become clear that genes that code for small stable RNAs, e.g., U1, U2 snRNA, and the U3 snoRNA, are often found adjacent to coiled bodies. High concentrations of transcription factors and RNA processing factors in and around coiled bodies indicate that they may be involved in the expression of the adjacent genes. In order to investigate whether coiled bodies could play a role in the synthesis of intron-encoded snoRNAs the distribution of coiled bodies was studied relative to three different snoRNA host genes, i.e., hsc70, RPS3, UHG. All three were found adjacent to coiled bodies at significantly high frequencies (11-19%), compared to control sequences (0-2%), to conclude a preferential association between the snoRNA host genes and coiled bodies. This association could point to a possible role for coiled bodies in the synthesis and/or maturation of snoRNAs. An involvement in snoRNA production could explain the presence of transcription factors, splicing factors, and fibrillarin in coiled bodies.
