Department of Botany, Canadian Institute for Advanced Research, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.
Proc Natl Acad Sci U S A. 2012 Jul 31;109(31):12638-43. doi: 10.1073/pnas.1205020109. Epub 2012 Jul 16.
Microsporidia of the genus Encephalitozoon are widespread pathogens of animals that harbor the smallest known nuclear genomes. Complete sequences from Encephalitozoon intestinalis (2.3 Mbp) and Encephalitozoon cuniculi (2.9 Mbp) revealed massive gene losses and reduction of intergenic regions as factors leading to their drastically reduced genome size. However, microsporidian genomes also have gained genes through horizontal gene transfers (HGT), a process that could allow the parasites to exploit their hosts more fully. Here, we describe the complete sequences of two intermediate-sized genomes (2.5 Mbp), from Encephalitozoon hellem and Encephalitozoon romaleae. Overall, the E. hellem and E. romaleae genomes are strikingly similar to those of Encephalitozoon cuniculi and Encephalitozoon intestinalis in both form and content. However, in addition to the expected expansions and contractions of known gene families in subtelomeric regions, both species also were found to harbor a number of protein-coding genes that are not found in any other microsporidian. All these genes are functionally related to the metabolism of folate and purines but appear to have originated by several independent HGT events from different eukaryotic and prokaryotic donors. Surprisingly, the genes are all intact in E. hellem, but in E. romaleae those involved in de novo synthesis of folate are all pseudogenes. Overall, these data suggest that a recent common ancestor of E. hellem and E. romaleae assembled a complete metabolic pathway from multiple independent HGT events and that one descendent already is dispensing with much of this new functionality, highlighting the transient nature of transferred genes.
微孢子虫属的脑炎微孢子虫是广泛存在的动物病原体，它们拥有已知最小的核基因组。来自脑炎微孢子虫（2.3 Mbp）和兔脑炎微孢子虫（2.9 Mbp）的完整序列揭示了大规模的基因缺失和基因间区的减少是导致其基因组大小急剧缩小的因素。然而，微孢子虫的基因组也通过水平基因转移（HGT）获得了基因，这一过程可以使寄生虫更充分地利用其宿主。在这里，我们描述了两种中等大小基因组（2.5 Mbp）的完整序列，来自脑炎微孢子虫和脑炎微孢子虫。总的来说，E. hellem 和 E. romaleae 的基因组在形式和内容上与兔脑炎微孢子虫和脑炎微孢子虫的基因组非常相似。然而，除了在端粒区域预期的已知基因家族的扩张和收缩之外，这两个物种还发现了许多不在其他微孢子虫中发现的编码蛋白的基因。所有这些基因在功能上都与叶酸和嘌呤的代谢有关，但似乎是通过来自不同真核生物和原核生物供体的几个独立的 HGT 事件起源的。令人惊讶的是，这些基因在 E. hellem 中都是完整的，但在 E. romaleae 中，参与叶酸从头合成的基因都是假基因。总的来说，这些数据表明，E. hellem 和 E. romaleae 的最近共同祖先从多个独立的 HGT 事件中组装了一个完整的代谢途径，而一个后代已经放弃了大部分新功能，突出了转移基因的短暂性。
