Paquin B, Laforest M J, Lang B F
Program in Evolutionary Biology, Canadian Institute for Advanced Research, Département de Biochimie, Université de Montréal, Montreal, Quebec, Canada.
Mol Biol Evol. 2000 Nov;17(11):1760-8. doi: 10.1093/oxfordjournals.molbev.a026274.
The mitochondrial DNA (mtDNA) of the chytridiomycete fungus Allomyces macrogynus contains 81 G+C-rich sequence elements that are 26-79 bases long and can be folded into a unique secondary structure consisting of two stem-loops. At the primary sequence level, the conservation of these double-hairpin elements (DHEs) is variable, ranging from marginal to complete identity. Forty of these DHEs are inserted in intergenic regions, 35 in introns, and 6 in variable regions of rRNA genes. Ten DHEs are inserted into other DHE elements (twins); two even form triplets. A comparison of DHE sequences shows that loop regions contain more sequence variation than helical regions and that the latter often contain compensatory base changes. This suggests a functional importance of the DHE secondary structure. We further identified nine DHEs in a 4-kb region of Allomyces arbusculus, a close relative of A. macrogynus. Eight of these DHEs are highly similar in sequence (90%-100%) to those in A. macrogynus, but only five are inserted at the same positions as in A. macrogynus. Interestingly, DHEs are also found in the mtDNAs of other chytridiomycetes, as well as certain zygomycete and ascomycete fungi. The overall distribution pattern of DHEs in fungal mtDNAs suggests that they are mobile elements.
壶菌纲真菌大雌异水霉的线粒体DNA（mtDNA）包含81个富含G+C的序列元件，长度为26至79个碱基，可折叠成由两个茎环组成的独特二级结构。在一级序列水平上，这些双发夹元件（DHE）的保守性各不相同，从勉强可辨到完全相同。其中40个DHE插入基因间隔区，35个插入内含子，6个插入rRNA基因的可变区。10个DHE插入到其他DHE元件中（形成双联体）；甚至有两个形成三联体。对DHE序列的比较表明，环区的序列变异比螺旋区更多，而且后者常常含有补偿性碱基变化。这表明DHE二级结构具有功能重要性。我们在大雌异水霉的近缘种小枝异水霉的一个4 kb区域中进一步鉴定出9个DHE。其中8个DHE在序列上与大雌异水霉中的DHE高度相似（90%-100%），但只有5个与大雌异水霉中的插入位置相同。有趣的是，在其他壶菌纲真菌以及某些接合菌纲和子囊菌纲真菌的mtDNA中也发现了DHE。DHE在真菌mtDNA中的总体分布模式表明它们是可移动元件。
