Dryden D T, Cooper L P, Thorpe P H, Byron O
Institute of Cell & Molecular Biology, University of Edinburgh, U.K.
Biochemistry. 1997 Feb 4;36(5):1065-76. doi: 10.1021/bi9619435.
Type I DNA restriction/modification enzymes protect the bacterial cell from viral infection by cleaving foreign DNA which lacks N6-adenine methylation within a target sequence and maintaining the methylation of the targets on the host chromosome. It has been noted that the genes specifying type I systems can be transferred to a new host lacking the appropriate, protective methylation without any adverse effect. The modification phenotype apparently appears before the restriction phenotype, but no evidence for transcriptional or translational control of the genes and the resultant phenotypes has been found. Type I enzymes contain three types of subunit, S for sequence recognition, M for DNA modification (methylation), and R for DNA restriction(cleavage), and can function solely as a M2S1 methylase or as a R2M2S1 bifunctional methylase/nuclease. We show that the methylase is not stable at the concentrations expected to exist in vivo, dissociating into free M subunit and M1S1, whereas the complete nuclease is a stable structure. The M1S1 form can bind the R subunit as effectively as the M2S1 methylase but possesses no activity; therefore, upon establishment of the system in a new host, we propose that most of the R subunit will initially be trapped in an inactive complex until the methylase has been able to modify and protect the host chromosome. We believe that the in vitro assembly pathway will reflect the in vivo situation, thus allowing the assembly process to at least partially explain the observations that the modification phenotype appears before the restriction phenotype upon establishment of a type I system in a new host cell.
I型DNA限制/修饰酶通过切割在靶序列内缺乏N6 - 腺嘌呤甲基化的外源DNA，并维持宿主染色体上靶标的甲基化，来保护细菌细胞免受病毒感染。值得注意的是，指定I型系统的基因可以转移到缺乏适当保护性甲基化的新宿主中，而不会产生任何不利影响。修饰表型显然在限制表型之前出现，但尚未发现有关基因及其产生的表型的转录或翻译控制的证据。I型酶包含三种亚基，S用于序列识别，M用于DNA修饰（甲基化），R用于DNA限制（切割），并且可以仅作为M2S1甲基化酶或作为R2M2S1双功能甲基化酶/核酸酶发挥作用。我们发现，甲基化酶在预期存在于体内的浓度下不稳定，会解离成游离的M亚基和M1S1，而完整的核酸酶是一种稳定的结构。M1S1形式可以像M2S1甲基化酶一样有效地结合R亚基，但没有活性；因此，在新宿主中建立该系统后，我们提出大多数R亚基最初将被困在无活性的复合物中，直到甲基化酶能够修饰并保护宿主染色体。我们认为体外组装途径将反映体内情况，从而使组装过程至少部分地解释在新宿主细胞中建立I型系统时修饰表型在限制表型之前出现的观察结果。
