Nuclear Speckles enable processing of RNA from GC-rich Isochores [SON_Halo_CR_H3K9_H327me3]

Nuclear Speckles are large, multi-component condensates in the mammalian nuclei that are thought to be involved in various stages of gene expression, including transcription, splicing, and export. Investigating the direct role of Speckles in these processes have been intractable due to their heterogeneous, multi-component nature. Here we show that Speckles are necessary for the splicing of GC-rich, levelled intron/exon architecture. Acute removal of Speckles leads to a drastic downregulation of GC-rich genes which are clustered within GC-rich isochores. Genes outside these domains are physically away from Speckles and are not affected from loss of Speckles. Animal genomes that do not have GC-rich isochores, do not have GC-rich levelled intron/exon architecture and do not have Speckles. Our results suggest that the expansion of Speckles core proteins’ intrinsically disordered domains during vertebrate evolution facilitated GC-content elevation in Amniotes by creating a condensate necessary to splice the by-products of this process: GC-rich, levelled intron/exon architecture.