Lane Christopher E, van den Heuvel Krystal, Kozera Catherine, Curtis Bruce A, Parsons Byron J, Bowman Sharen, Archibald John M
Canadian Institute for Advanced Research, Integrated Microbial Biodiversity Program, Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, NS, Canada B3H 1X5.
Proc Natl Acad Sci U S A. 2007 Dec 11;104(50):19908-13. doi: 10.1073/pnas.0707419104. Epub 2007 Dec 6.
Nucleomorphs are the remnant nuclei of algal endosymbionts that took up residence inside a nonphotosynthetic eukaryotic host. The nucleomorphs of cryptophytes and chlorarachniophytes are derived from red and green algal endosymbionts, respectively, and represent a stunning example of convergent evolution: their genomes have independently been reduced and compacted to <1 megabase pairs (Mbp) in size (the smallest nuclear genomes known) and to a similar three-chromosome architecture. The molecular processes underlying genome reduction and compaction in eukaryotes are largely unknown, as is the impact of reduction/compaction on protein structure and function. Here, we present the complete 0.572-Mbp nucleomorph genome of the cryptophyte Hemiselmis andersenii and show that it is completely devoid of spliceosomal introns and genes for splicing RNAs-a case of complete intron loss in a nuclear genome. Comparison of H. andersenii proteins to those encoded in the slightly smaller (0.551-Mbp) nucleomorph genome of another cryptophyte, Guillardia theta, and to their homologs in the unicellular red alga Cyanidioschyzon merolae reveal that (i) cryptophyte nucleomorph genomes encode proteins that are significantly smaller than those in their free-living algal ancestors, and (ii) the smaller, more compact G. theta nucleomorph genome encodes significantly smaller proteins than that of H. andersenii. These results indicate that genome compaction can eliminate both coding and noncoding DNA and, consequently, drive the evolution of protein structure and function. Nucleomorph proteins have the potential to reveal the minimal functional units required for basic eukaryotic cellular processes.
核质体是藻类内共生体的残余细胞核，这些内共生体曾寄居于非光合真核宿主细胞内。隐藻和绿藻门的核质体分别源自红藻和绿藻内共生体，是趋同进化的一个惊人例证：它们的基因组已独立缩减并压缩至小于1兆碱基对（Mbp）（已知最小的核基因组），且具有相似的三条染色体结构。真核生物中基因组缩减和压缩背后的分子过程在很大程度上尚不清楚，缩减/压缩对蛋白质结构和功能的影响也不清楚。在此，我们展示了隐藻安德逊半片藻完整的0.572 Mbp核质体基因组，并表明它完全没有剪接体内含子和用于剪接RNA的基因——这是核基因组中完全内含子缺失的一个例子。将安德逊半片藻的蛋白质与另一种隐藻——嗜热四爿藻稍小（0.551 Mbp）的核质体基因组中编码的蛋白质，以及它们在单细胞红藻梅氏蓝纤维藻中的同源物进行比较，结果表明：（i）隐藻核质体基因组编码的蛋白质明显小于其自由生活的藻类祖先所编码的蛋白质；（ii）更小、更紧凑的嗜热四爿藻核质体基因组编码的蛋白质比安德逊半片藻的明显更小。这些结果表明，基因组压缩既能消除编码DNA，也能消除非编码DNA，从而推动蛋白质结构和功能的进化。核质体蛋白质有潜力揭示基本真核细胞过程所需的最小功能单元。
