Splicing segregation: the minor spliceosome acts outside the nucleus and controls cell proliferation.

The functional relevance and the evolution of two parallel mRNA splicing systems in eukaryotes--a major and minor spliceosome that differ in abundance and splicing rate--are poorly understood. We report here that partially spliced pre-mRNAs containing minor-class introns undergo nuclear export and that minor-class snRNAs are predominantly cytoplasmic in vertebrates. Cytoplasmic interference with the minor spliceosome further indicated its functional segregation from the nucleus. In keeping with this, minor splicing was only weakly affected during mitosis. By selectively interfering with snRNA function in zebrafish development and in mammalian cells, we revealed a conserved role for minor splicing in cell-cycle progression. We argue that the segregation of the splicing systems allows for processing of partially unspliced cytoplasmic transcripts, emerging as a result of different splicing rates. The segregation offers a mechanism accounting for spliceosome evolution in a single lineage and provides a means for nucleus-independent control of gene expression.