Poxvirus DNA Replication

痘病毒编码在感染细胞的细胞质内复制其基因组所需的酶和因子。痘苗病毒是痘病毒家族的典型成员，它为研究DNA复制机制提供了一种结合生化和遗传方法的独特系统。DNA被合成为串联体，在病毒组装过程中被分解成单位长度的基因组并打包。对条件致死突变体的研究表明，VACV的DNA复制需要5种早期蛋白：E9 DNA聚合酶、D4尿嘧啶DNA糖基酶、A20加工因子、B1蛋白激酶和D5核苷三磷酸酶(NTPase)。该DNA聚合酶催化依赖于引物和模板的合成，并具有3-5个核外溶解活性。D4在DNA复制中的重要作用不依赖于其尿嘧啶DNA糖基酶活性，该酶可能具有兼性修复功能。A20和D4蛋白相互作用，共同为DNA聚合酶提供加工能力。最近发现，B1K可以磷酸化一种叫做BAF的细胞DNA结合蛋白，并阻止后者阻止VACV的DNA复制。条件致死D5突变体的快速停止DNA复制表型提示了复制分叉的功能。D5还与A20相互作用，形成多聚体。D5的潜在作用来自广泛的蛋白质序列分析，这些分析表明，90 kDa的D5蛋白是NTPase AAA类中解旋酶超家族III的成员，包括其他一些DNA和RNA病毒的复制型解旋酶。

Poxviruses encode enzymes and factors needed for replication of their genomes within the cytoplasm of infected cells. Vaccinia virus, the prototypic member of the poxvirus family, provides a unique system for combining biochemical and genetic approaches for investigating mechanisms of DNA replication. The DNA is synthesized as concatemers that are resolved into unit length genomes and packaged during virus assembly. Studies with conditional lethal mutants indicate that five VACV early proteins are required for DNA replication: namely E9 DNA polymerase, D4 uracil DNA glycosylase, A20 processivity factor, B1 protein kinase and D5 nucleoside triphosphatase (NTPase). The DNA polymerase catalyzes primer- and template-dependent synthesis and possesses 3 to 5 exonucleolytic activity. The essential role of D4 in DNA replication is independent of its uracil DNA glycosylase activity, which presumably has a facultative repair function. The A20 and D4 proteins interact and together provide processivity for the DNA polymerase. The B1 kinase was recently shown to phosphorylate a cellular DNA-binding protein called BAF and prevent the latter from blocking VACV DNA replication. The fast stop DNA replication phenotype of conditional lethal D5 mutants suggested a function at the replication fork. D5 also interacts with A20 and forms multimers. Potential roles for D5 have come from extensive protein sequence analyses, which indicate that the 90-kDa D5 protein is a member of the helicase superfamily III within the AAA+ class of NTPases, which includes the replicative helicases of some other DNA and RNA viruses.