What evidence supports distinct subdomains for RNA processing and export in the nucleus?

• A. There is no evidence for distinct subdomains; RNA processing and export occur uniformly.
• B. The presence of nuclear bodies (speckles, Cajal bodies) and the localization of processing factors near transcription sites and NPCs indicate functional subdomains for processing and export.
• C. Subdomains exist only in diseased cells.
• D. Processing occurs only in the cytoplasm.

Answer: (B)

RNA processing and export happen in organized subnuclear zones that bring together the right machinery exactly where a transcript is made or readied for export. Nuclear speckles act as dynamic hubs rich in splicing factors, and they tend to associate with transcription sites, which allows nascent RNAs to be processed quickly as they are synthesized. Cajal bodies are another specialized domain where components needed to assemble and mature small nuclear ribonucleoproteins (snRNPs) gather, ensuring that the processing toolkit is ready for action. The close proximity of these processing factors to transcription factories and to nuclear pore complexes creates a spatial workflow: transcripts are processed in nearby nuclear domains and then handed off to export routes at the nuclear envelope. This organization is supported by observations that processing factors localize to these bodies, transcripts traffic between transcription sites, processing domains, and export channels, and disruptions to these compartments impair RNA maturation and export.

So this arrangement—distinct nuclear bodies and strategic localization of processing and export components—provides clear evidence for functional subdomains within the nucleus.