西亞試劑：Origin and Evolution of the Eukaryotic SSU Processome Revea

Origin and Evolution of the Eukaryotic SSU Processome Revealed by a Comprehensive Genomic Analysis and Implications for the Origin of the Nucleolus

Jin-Mei Feng1，2， Hai-Feng Tian1，2 and Jian-Fan Wen1，*

As a nucleolar complex for small-subunit ribosomal RNA processing， small-subunit (SSU) processome has been extensively studied mainly in Saccharomyces cerevisiae， but not in diverse organisms， leaving open the questions of whether it is a ubiquitous mechanism across eukaryotes and how it evolved in the course of the evolution of eukaryotes. Genome-wide survey and identification of SSU processome components showed that the majority of all the 77 yeast SSU processome proteins possess homologs in almost all of the main eukaryotic lineages， and 14 of them have homologs in archaea but few in bacteria， suggesting that the complex is ubiquitous in eukaryotes， and its evolutionary history began with abundant protein homologues being present in archaea and then a fairly complete form of the complex emerged in the last eukaryotic common ancestor (LECA). Phylogenetic analysis indicated that ancient gene duplication and functional divergence of the protein components of the complex occurred frequently during the evolutionary origin of the LECA from prokaryotes. We found that such duplications not only increased the complex's components， but also produced some new functional proteins involved in other nucleolar functions such as ribosome biogenesis and even some non-nucleolar (but nuclear) proteins participating in pre-mRNA splicing， implying the evolutionary emergence of the sub-nuclear compartment -- the nucleolus， has occurred in the LECA. Therefore， the LECA harbored not only complicated SSU processomes but even a nucleolus. Our analysis also revealed that gene duplication， innovation， and loss， caused further divergence of the complex during the divergence of eukaryotes.

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