Evolution of an RNP assembly system: A minimal SMN complex facilitates formation of UsnRNPs in Drosophila melanogaster

In vertebrates, assembly of spliceosomal uridine-rich small nuclearribonucleoproteins (UsnRNPs) is mediated by the SMN complex, amacromolecular entity composed of the proteins SMN and Gemins2–8. Here we have studied the evolution of this machinery usingcomplete genome assemblies of multiple model organisms. TheSMN complex has gained complexity in evolution by a blockwiseaddition of Gemins onto an ancestral core complex composed ofSMN and Gemin2. In contrast to this overall evolutionary trend tomore complexity in metazoans, orthologs of most Gemins aremissing in dipterans. In accordance with these bioinformatic dataa previously undescribed biochemical purification strategy eluci-dated that the dipteran