Shaping up genomes: Prokaryotic roots and eukaryotic diversification of SMC complexes

Abstract

SummaryAcross the tree of life, SMC complexes organize, segregate and regulate DNA. In contrast to prokaryotes, which often possess one SMC complex, eukaryotes usually have four such complexes. This expanded set is involved in managing the considerably larger nuclear genomes of eukaryotes, which are distributed across multiple chromosomes. Despite their essential functions, SMC complexes exhibit variations across model eukaryotes, suggesting an even greater variety of these complexes that remains unexplored. Here, we aimed to uncover the diversity and evolution of SMC complexes across eukaryotes, and their deeper, prokaryotic evolutionary origins. For this, we conducted in-depth comparative genomic and phylogenetic analyses of SMC complexes. We show that the last eukaryotic common ancestor (LECA) likely had fully-fledged versions of all four complexes and that the condensin II complex was later lost at least 30 times in eukaryotes. We report evidence that proteins previously designated as functional analogs in various model organisms (e.g., Sororin, Securin, Nse5 and Nse6) are in fact genuine orthologs. Finally, we traced the prokaryotic origins of these complexes and propose that a single SMC complex duplicated in an early archaeon. Altogether, we provide a comprehensive overview of eukaryotic SMC complex diversity and evolution, both addressing and generating questions about their functioning in ancestral and contemporary organisms.