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|作者:||K. Y. Lee|
B. Y. Chang
|摘要:||限制酶可分為第一限制酶，第二限制酶以及第三限制酶等三種。其實第一類限制酶的分子量為450,000-750,000之關係由三個次單位組而成。次單位之ㄧ為hsd R基因的產物，專司外來之DNA的剪切。hsd M 基因的產物專司自身或者外來DNA的修飾，而 hsd S 基因產物則掌理DNA的識別。該酶的分布，據目前學界的研究，僅侷信於腸細菌科的微生物。其在菌體的功用在於防禦他DNA的入侵。
Three types of restriction enzymes are recognized today. They are Type Ⅰ, Type Ⅱ and Type Ⅲ enzymes. Each of the Type Ⅰ enzyme, with a molecular weight around 451,000-750,000, is composed of three subunits, the latter are encoded by three separats genes. The product of hsd R gene is responsible for foreign DNA restriction. The product of hsd M gene engages in DNA methylation, and the procuct of hsd S gene specializes in DNA recognition. The Type Ⅰ enzymes, to date, are found only in Entero-bacteriaceae, they are key components of the cell defense mechanism. Like Type Ⅰ enzymes, the Type Ⅲ restriction enzymes are also involved in defense systems. The individual enzyme, with a molecular weight about 200,000, consists of two subunits. One of the two has the restroction activity and another the duall functions of methylation and recognition. The enzymes so far studied are integral parts of the phage P1 and plosmid P15 resistance arsenal. In comparison with the enzymes mentioned above, Type Ⅱ restriction enzymes are comsiderably less complex and quite a bit smaller. They are close to one tenth the size of their larger relatives in terms of molecular weight. The restriction activity and methylation activisity are separate proteins, each of which is capable of recognizing the target DNA. The enzymes are widespread, they have been found in hundreds of microbes. Their real function in cell is far from being clear. However, their highly specific DNA cleavage ability has made them the indispenable tools of genetic engineering.
|Appears in Collections:||第11期|
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