"Splicing speckles" are major nuclear domains rich in components of the splicing 
machinery and polyA(+) RNA. Although speckles contain little detectable 
transcriptional activity, they are found preferentially associated with specific 
mRNA-coding genes and gene-rich R bands, and they accumulate some unspliced 
pre-mRNAs. RNA polymerase II transcribes mRNAs and is required for splicing, 
with some reports suggesting that the inactive complexes are stored in splicing 
speckles. Using ultrathin cryosections to improve optical resolution and 
preserve nuclear structure, we find that all forms of polymerase II are present, 
but not enriched, within speckles. Inhibition of polymerase activity shows that 
speckles do not act as major storage sites for inactive polymerase II complexes 
but that they contain a stable pool of polymerase II phosphorylated on serine(2) 
residues of the C-terminal domain, which is transcriptionally inactive and may 
have roles in spliceosome assembly or posttranscriptional splicing of pre-mRNAs. 
Paraspeckle domains lie adjacent to speckles, but little is known about their 
protein content or putative roles in the expression of the speckle-associated 
genes. We find that paraspeckles are transcriptionally inactive but contain 
polymerase II, which remains stably associated upon transcriptional inhibition, 
when paraspeckles reorganize around nucleoli in the form of caps.