Poxvirus Gene Expression

痘病毒在受感染细胞的细胞质中编码转录和复制其基因组所需的酶和因子。痘苗病毒是痘病毒家族的原型成员，为研究基因调控和mRNA生物合成机制提供了一个独特的生物化学和遗传方法相结合的系统。对牛痘病毒的研究表明，这些基因在时间上可分为早期、中期和晚期三种类型。每个基因类都有一个一致的DNA启动子序列和相应的转录因子，这些转录因子与病毒编码的多亚基RNA聚合酶相互作用。早期基因的转录系统在感染病毒颗粒组装时被包装在其内部，而中、晚期基因转录因子则在感染后陆续合成，并定位于细胞质工厂区域内。痘病毒还编码酶，通过在5‘端添加一个帽状结构和在3’端添加一个聚(a)尾来修饰它们的mRNA，这是有效翻译和稳定性所必需的。细胞蛋白质合成的关闭和病毒蛋白质合成的严格调控是由切割帽状结构的痘病毒酶调控的。

Poxviruses encode enzymes and factors needed for transcription and 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 gene regulation and mRNA biosynthesis. Studies with vaccinia virus indicated that the genes are divided into three temporal classes - early, intermediate and late. Each gene class has a consensus DNA promoter sequence and corresponding transcription factors that interact with the virus-encoded multisubunit RNA polymerase. The transcription system for early genes is packaged within the infectious virus particle during its assembly, whereas the factors for intermediate and late gene transcription are synthesized successively after infection and localize within cytoplasmic factory areas. Poxviruses also encode enzymes that modify their mRNA by adding a cap structure to the 5' end and a poly(A) tail to the 3' end, which are necessary for efficient translation and stability. The shut down of cellular protein synthesis and the tight regulation of viral protein synthesis are regulated by poxvirus enzymes that cleave the cap structure.