Snurposomes and coiled bodies.

In the GV of amphibian oocytes, the splicing snRNPs (U1, U2, U4, U5, and U6) occur on most of the lampbrush chromosome loops in association with the nascent transcripts. They also occur in thousands of small extrachromosomal bodies (1-4 microns in diameter) designated B snurposomes. U7 snRNA, which is involved in processing the 3' end of histone pre-mRNAs, occurs in a few dozen extrachromosomal bodies (1-20 microns in diameter) called C snurposomes. C snurposomes often have B snurposomes attached to their surface and B-like inclusions; these compound structures are known as spheres or sphere organelles. One or two sphere organelles are attached to the lampbrush chromosomes at the histone gene loci. Two snRNAs (or snoRNAs) known to be involved in pre-rRNA processing (U3 and U8) occur in the 1000 or so extrachromosomal nucleoli of the GV. We looked for a snurposome that might contain U3 and U8 but not rDNA or rRNA. We were unable to find such a snurposome, but we did identify a hitherto unrecognized population of minute nucleoli in the size range of B snurposomes. Prenucleolar bodies in telophase/early interphase nuclei meet the definition of a pre-rRNA snurposome in that they contain U3 snoRNA and fibrillarin (and probably other processing components) but lack rDNA and do not synthesize rRNA. The structures previously identified as prenucleolar bodies in pronuclei formed in vitro in Xenopus egg extracts share many components with coiled bodies from HeLa nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)