Tanabe Yuuhiko, Yokota Akira, Sugiyama Junta
Institute of Molecular and Cellular Biosciences, The University of Tokyo, Tokyo, Japan.
J Mol Evol. 2002 May;54(5):692-702. doi: 10.1007/s00239-001-0071-4.
The origins of fungal group I introns within nuclear small-subunit (nSSU) rDNA are enigmatic. This is partly because they have never been reported in basal fungal phyla (Zygomycota and Chytridiomycota), which are hypothesized to be ancestral to derived phyla (Ascomycota and Basidiomycota). Here we report group I introns from the nSSU rDNA of two zygomycete fungi, Zoophagus insidians (Zoopagales) and Coemansia mojavensis (Kickxellales). Secondary structure analyses predicted that both introns belong to the IC1 subgroup and that they are distantly related to each other, which is also suggested by different insertion sites. Molecular phylogenetic analyses indicated that the IC1 intron of Z. insidians is closely related to the IC1 intron inserted in the LSU rDNA of the basidiomycete fungus Clavicorona taxophila, which strongly suggests interphylum horizontal transfer. The IC1 intron of C. mojavensis has a low phylogenetic affinity to other fungal IC1 introns inserted into site 943 of nSSU rDNA (relative to E. coli 16S rDNA). It is noteworthy that this intron contains a putative ORF containing a His-Cys box motif in the antisense strand, a hallmark for nuclear-encoded homing endonucleases. Overall, molecular phylogenetic analyses do not support the placement of these two introns in basal fungal IC1 intron lineages. This result leads to the suggestion that fungal IC1 introns might have invaded or been transferred laterally after the divergence of the four major fungal phyla.
核小亚基（nSSU）核糖体DNA中真菌I组内含子的起源尚不清楚。部分原因是它们从未在基部真菌门（接合菌门和壶菌门）中被报道过，据推测这些基部真菌门是衍生真菌门（子囊菌门和担子菌门）的祖先。在此，我们报道了来自两种接合菌真菌——内食噬菌虫（虫霉目）和莫哈韦科曼氏菌（梳霉目）的nSSU核糖体DNA中的I组内含子。二级结构分析预测，这两个内含子都属于IC1亚组，且彼此关系较远，不同的插入位点也表明了这一点。分子系统发育分析表明，内食噬菌虫的IC1内含子与插入担子菌嗜木生棒孢菌大亚基（LSU）核糖体DNA中的IC1内含子密切相关，这强烈表明存在跨门水平转移。莫哈韦科曼氏菌的IC1内含子与插入nSSU核糖体DNA第943位点（相对于大肠杆菌16S核糖体DNA）的其他真菌IC1内含子的系统发育亲和力较低。值得注意的是，这个内含子在反义链中包含一个推定的开放阅读框，其中含有一个组氨酸 - 半胱氨酸框基序，这是核编码归巢内切核酸酶的一个标志。总体而言，分子系统发育分析不支持将这两个内含子置于基部真菌IC1内含子谱系中。这一结果表明，真菌IC1内含子可能在四大真菌门分化之后侵入或横向转移。
