Keeling Patrick J, Fast Naomi M, Law Joyce S, Williams Bryony A P, Slamovits Claudio H
Canadian Institute for Advanced Research, Botany Department, University of British Columbia, 3529-6270 University Boulevard, Vancouver, BC V6T 1Z4, Canada.
Folia Parasitol (Praha). 2005 May;52(1-2):8-14. doi: 10.14411/fp.2005.002.
Microsporidia have been known for some time to possess among the smallest genomes of any eukaryote. There is now a completely sequenced microsporidian genome, as well as several other large-scale sequencing efforts, so the nature of these genomes is becoming apparent. This paper reviews some of the characteristics of microsporidian genomes in general, and some of the recent discoveries made through comparative genomic analyses. In general, microsporidian genomes are both reduced and compacted. Reduction takes place through gene loss, which is understandable in obligate intracellular parasites that rely on their host for many metabolites. Compaction is a more complex process, and is as yet not fully understood. It is clear from genomes surveyed thus far that the remaining genes are tightly packed and that there is little non-coding sequence, resulting in some extraordinary arrangements, including overlapping genes. Compaction also seems to affect certain aspects of genome evolution, like the frequency of rearrangements. The force behind this compaction is not known, and is especially interesting in light of the fact that surveys of genomes that are significantly different in size yield similar complements of protein-coding genes. There are some interesting exceptions, including catalase, photolyase and some mitochondrial proteins, but the rarity of these raises an interesting question as to what accounts for the significant differences seen in the genome sizes among microsporidia.
一段时间以来，人们已知微孢子虫拥有任何真核生物中最小的基因组之一。目前已有一个完全测序的微孢子虫基因组，以及其他一些大规模测序工作，因此这些基因组的性质正变得清晰起来。本文综述了微孢子虫基因组的一些总体特征，以及通过比较基因组分析取得的一些最新发现。一般来说，微孢子虫基因组既存在基因减少现象，又呈现出紧凑的特点。基因减少是通过基因丢失实现的，这在依赖宿主获取多种代谢物的专性细胞内寄生虫中是可以理解的。基因组紧凑化是一个更为复杂的过程，目前尚未完全了解。从迄今为止所研究的基因组来看，很明显剩余的基因紧密排列，非编码序列很少，这导致了一些非同寻常的排列方式，包括重叠基因。基因组紧凑化似乎也会影响基因组进化的某些方面，比如重排频率。这种紧凑化背后的驱动力尚不清楚，鉴于对大小差异显著的基因组进行调查却产生相似的蛋白质编码基因互补情况，这一点尤其令人感兴趣。存在一些有趣的例外情况，包括过氧化氢酶、光解酶和一些线粒体蛋白，但这些情况很少见，这就引发了一个有趣的问题：究竟是什么导致了微孢子虫基因组大小的显著差异。
