Interactions of U2 gene loci and their nuclear transcripts with Cajal (coiled) bodies: Evidence for PreU2 within Cajal bodies

The Cajal (coiled) body (CB) is a structure enriched in proteins involved i n mRNA, rRNA, and snRNA metabolism. CBs have been shown to interact with sp ecific histone and snRNA gene loci. To examine the potential role of CBs in U2 snRNA metabolism, we used a variety of genomic and oligonucleotide prob es to visualize in situ newly synthesized U2 snRNA relative to U2 loci and CBs. Results demonstrate that long spacer sequences between U2 coding repea ts are transcribed, supporting other recent evidence that U2 transcription proceeds past the 3' box. The presence of bright foci of this U2 locus RNA differed between alleles within the same nucleus; however, this did not cor relate with the loci's association with a CB. Experiments with specific oli gonucleotide probes revealed signal fur preU2 RNA within CBs. PreU2 was als o defected in the locus-associated RNA foci, whereas sequences 3' of preU2 were found only in these foci, not in CBs. This suggests that a longer prim ary transcript is processed before entry into CBs. Although this work shows that direct contact of a U2 locus with a CB is not simply correlated with RNA at that locus, it provides the first evidence of new preU2 transcripts within CBs. We also show that, in contrast to CBs, SMN gems do not associat e with U2 gene loci and do not contain preU2. Because other evidence indica tes that preU2 is processed in the cytoplasm before assembly into snRNPs, r esults point to an involvement of CBs in modification or transport of preU2 RNA.