Janosi L, Yonemitsu H, Hong H, Kaji A
Department of Microbiology, School of Medicine, University of Pennsylvania, Philadelphia 19104.
J Mol Biol. 1994 Sep 9;242(1):45-61. doi: 10.1006/jmbi.1994.1556.
The kanamycin resistance plasmid Rts1 restricts the growth of bacteriophage T2, T4 and T6. The DNA of these phage contains hydroxymethylcytosine (HMC) in place of regular cytosine and is modified by glucosylation. When HMC is not glucosylated, as in the DNA of glucosyl transferase-deficient T4 phage, this restriction becomes less apparent, a phenomenon not observed with any other known restriction systems. On the other hand, glucosylation of HMC in T6 phage leads to a less efficient restriction, while restriction of bacteriophage T2 remains unchanged. The modulating effect of glucose cannot be seen when cells contain a large amount of this enzyme, as in the case when multiple copies of its determinant are present in the cells. T-odd phage and bacteriophage lambda are not restricted by Rts1 suggesting that the restriction is specific to DNA containing HMC. The restriction phenotype is due to a single gene coding for a polypeptide of 293 amino acids. This enzyme has been named PvuRts1I. A gene with the sequence motifs similar to modification enzymes was found upstream of the gene coding for PvuRts1I. This gene, however, neither modifies the restriction phenotype of PvuRts1I, nor codes for detectable modification enzyme. T4 mutants with increased resistance to PvuRts1I appear to have deficiency in their beta-glucosyl transferase enzyme.
卡那霉素抗性质粒Rts1可限制噬菌体T2、T4和T6的生长。这些噬菌体的DNA含有羟甲基胞嘧啶（HMC）以取代常规胞嘧啶，并通过糖基化进行修饰。当HMC未被糖基化时，如在缺乏糖基转移酶的T4噬菌体的DNA中，这种限制作用就不那么明显，这是任何其他已知限制系统都未观察到的现象。另一方面，T6噬菌体中HMC的糖基化导致限制作用效率降低，而噬菌体T2的限制作用保持不变。当细胞中含有大量这种酶时，如细胞中存在其决定簇的多个拷贝时，就看不到葡萄糖的调节作用。T奇数噬菌体和噬菌体λ不受Rts1的限制，这表明这种限制作用对含有HMC的DNA具有特异性。这种限制表型是由一个编码293个氨基酸的多肽的单一基因引起的。这种酶被命名为PvuRts1I。在编码PvuRts1I的基因上游发现了一个具有与修饰酶相似序列基序的基因。然而，这个基因既不改变PvuRts1I的限制表型，也不编码可检测到的修饰酶。对PvuRts1I具有增强抗性的T4突变体似乎其β-葡萄糖基转移酶存在缺陷。
