Cui L, Webb B A
Department of Entomology, University of Kentucky, Lexington 40546-0091, USA.
J Virol. 1997 Nov;71(11):8504-13. doi: 10.1128/JVI.71.11.8504-8513.1997.
Polydnaviruses (PDVs) are double-stranded DNA viruses with segmented genomes that replicate only in the oviducts of some species of parasitic wasps and are required for the successful parasitization of lepidopteran insects. PDV DNA segments are integrated in the genomes of their associated wasp hosts, and some are nested; i.e., smaller segments are produced from and largely colinear with larger segments. To determine the internal structure of nested viral segments, the first complete nucleotide sequence of a PDV genome segment and its integration locus was determined. By restriction mapping, Southern blot, and sequence analyses, we demonstrated that the Campoletis sonorensis PDV segment W is integrated into wasp genomic DNA. DNA sequence analysis revealed that proviral segment W terminates in two 1,185-bp direct long terminal repeats (LTRs) in the wasp chromosome, while only one LTR copy is present in the extrachromosomal (viral) W. The results suggest that terminal direct repeats are a general feature of PDV DNA segment integration but that the homology and size of the repeats can vary extensively. Segment W contains 12 imperfect direct repeats of six different types between 89 bp and 1.9 kbp with 65 to 90% homology. The orientation and structure of the repeats suggest that W itself may have arisen through sequence duplication and subsequent divergence. Mapping, hybridization, and sequence analyses of cloned R and M demonstrated that these segments are nested within segment W and that internal imperfect direct repeats of one type are implicated in the homologous intramolecular recombination events that generate segments R and M. Interestingly, segment nesting differentially increases the copy number of genes encoded by segment W, suggesting that the unusual genomic organization of PDVs may be directly linked to the unique functions of this virus in its obligate mutualistic association with parasitic wasps.
多分DNA病毒（PDVs）是双链DNA病毒，其基因组呈片段化，仅在某些寄生蜂物种的输卵管中复制，是成功寄生鳞翅目昆虫所必需的。PDV DNA片段整合在其相关黄蜂宿主的基因组中，有些是嵌套的；即较小的片段由较大的片段产生并与其基本共线。为了确定嵌套病毒片段的内部结构，测定了一个PDV基因组片段及其整合位点的首个完整核苷酸序列。通过限制性图谱分析、Southern印迹分析和序列分析，我们证明了桑氏 Campoletis sonorensis PDV片段W整合到了黄蜂基因组DNA中。DNA序列分析表明，前病毒片段W在黄蜂染色体中以两个1185 bp的直接长末端重复序列（LTRs）终止，而在染色体外（病毒）W中仅存在一个LTR拷贝。结果表明，末端直接重复是PDV DNA片段整合的一个普遍特征，但重复序列的同源性和大小可能有很大差异。片段W包含12个不同类型的不完美直接重复序列，长度在89 bp至1.9 kbp之间，同源性为65%至90%。这些重复序列的方向和结构表明，W本身可能是通过序列复制和随后的分化产生的。对克隆的R和M进行的图谱分析、杂交分析和序列分析表明，这些片段嵌套在片段W内，并且一种类型的内部不完美直接重复序列与产生片段R和M的同源分子内重组事件有关。有趣的是，片段嵌套差异地增加了片段W编码的基因拷贝数，这表明PDVs不寻常的基因组组织可能与其在与寄生蜂的专性互利共生关系中的独特功能直接相关。
