文昌鱼嘌呤代谢酶基因:结构与演化

The Genes Encoding Purine Metabolism Enzymes in Amphioxus:Structure and Evolution

嘌呤代谢是动物含氮废物排出体外的重要途径之一。嘌呤代谢终产物在不同动物各不相同。这是由于进化过程中,参与代谢1种或多种酶基因或者酶活性的丢失所致。本文从JGI网站佛罗里达文昌鱼基因组数据库中搜索文昌鱼参与嘌呤代谢主要5种酶(黄嘌呤脱氢酶、尿酸酶、尿囊素酶、尿囊酸酶、尿素酶)的基因,并且将它们与其他12种具有代表性且基因组序列已知的动物包括人、小鼠、狗、鸡、爪蟾、河豚、斑马鱼、海鞘、海胆、果蝇、线虫、海葵的基因结构进行比较。结果发现文昌鱼参与嘌呤代谢的酶基因有的和脊椎动物基因相似(黄嘌呤脱氢酶和尿酸酶),有的和脊椎动物与无脊椎动物的基因都不相似(尿囊酸酶),还有的和无脊椎动物海胆与海葵基因外显子个数与内含子相位都相似但外显子之间缺乏对应关系(尿素酶)。所有这些都支持文昌鱼代表脊椎动物始祖代表类型,同时兼具特化特征的观点。尿素酶基因存在于水生无脊椎动物中,而在陆生的果蝇和线虫中均未发现尿素酶基因,因而我们推测随着动物由水生向陆生进化,参与尿素分解的尿素酶基因可能在陆生动物中完全丢失。

Purine catabolism is one of the primary pathways in which nitrogenous waste is produced for excretion in animals. The end product of purine metabolism varies from species to species. This has been shown to be due to the loss of certain enzymes involved in the catabolic pathway in some organisms during evolution. In this study, we demonstrated the presence of genes encoding all the five key enzymes （xanthine dehydrogenase, uricase, allantoinase, allantoicase, urease） involved in purine metabolism in the basal chordate Branchiostoma floridae. Structural comparisons with the related genes from other 12 representative animals （Homo sapiens, Mus musculus, Canis familiaris, Gallus gallus, Danio rerio, Xenopus tropicalis, Fugu rubripes, Ciona intestinalis , Strongylocentrotus purpuratus , Drosophila melanogaster , Caenorhabditis elegans and Nematostella vectensis ） revealed that B. floridae genes encoding xanthine dehydrogenase and uricase are similar to those of vertebrates and invertebrates, while B. floridae gene coding for allantoicase is neither similar to the vertebrate counterpart nor to the invertebrate counterpart. On the other hand, the exon numbers and intron phases for B. floroidae gene encoding urease is more or less similar to those of the invertebrates S. purpuratus and N. vectensis, but they apparently lack corresponding relations. All this supports the notion that amphioxus is a representative basal chordate with distinct radiated characteristics. In addition, we found that the urease gene is present only in the aquatic animals such as B. floridae, S. purpuratus and N. vectensis but not in the terrestrial organisms including D. melanogaster and C. elegans, suggesting the loss of the gene in the terrestrial animals.