Highly transcribed DNA regions (green) surround mRNA-processing domains (blue).

On page 981, Shopland et al. identify a new level of nuclear organization that positions highly transcribed DNA and chromosome segments near proteins needed for mRNA maturation.

Splicing factors and other proteins that process newly made transcripts accumulate in nuclear speckles called SC-35 domains. Shopland et al. find that these domains associate with R-bands, which are cytologically visible, gene-rich, and highly transcribed chromosome regions. Transcripts from both linked and unlinked genes were found within a given SC-35 domain. This shared usage indicates that the domains are not simply spots of RNA metabolic factors at an especially active locus, but are rather organized domains that may promote efficient transcript processing. Clustering of genes with numerous individual components of the large transcription and mRNA processing complexes probably hastens complex assembly and thus increases the efficiency of mRNA maturation.

Certain hyperactive genes may be targeted to or nucleate SC-35 domain formation and then promote the association of the neighboring chromosomal region with the domain. The highly transcribed COL1A1 gene was consistently associated with a domain, although sequences within the same R-band also contacted the domain ∼80% of the time. The organization of DNA to optimize the association of active gene regions with SC-35 domains may be the best explanation yet for the evolutionary origins of R-bands versus gene-poor G-bands. ▪