Basu Vandana Purohit, Song Min, Gao Lu, Rigby Sean T, Hanson Mark Nils, Bambara Robert A
Department of Biochemistry & Biophysics, Box 712, University of Rochester Medical Center, 601 Elmwood Avenue, Rochester, NY 14642, USA.
Virus Res. 2008 Jun;134(1-2):19-38. doi: 10.1016/j.virusres.2007.12.017. Epub 2008 Feb 14.
Human immunodeficiency virus type 1 (HIV-1) and other retroviruses replicate through reverse transcription, a process in which the single stranded RNA of the viral genome is converted to a double stranded DNA. The virally encoded reverse transcriptase (RT) mediates reverse transcription through DNA polymerase and RNase H activities. Conversion of the plus strand RNA to plus/minus strand RNA/DNA hybrid involves a transfer of the growing DNA strand from one site on the genomic RNA to another. This is called minus strong-stop DNA transfer. Later synthesis of the second or plus DNA strand involves a second strand transfer, involving a similar mechanism as the minus strand transfer. A basic feature of the strand transfer mechanism is the use of the RT RNase H to remove segments of the RNA template strand from the growing DNA strand, freeing a single stranded region to anneal to the second site. Viral nucleocapsid protein (NC) functions to promote transfer by facilitating this strand exchange process. Two copies of the RNA genomes, sometimes non-identical, are co-packaged in the genomes of retroviruses. The properties of the reverse transcriptase allow a transfer of the growing DNA strand between these genomes to occur occasionally at any point during reverse transcription, producing recombinant viral progeny. Recombination promotes structural diversity of the virus that helps it to survive host immunity and drug therapy. Recombination strand transfer can be forced by a break in the template, or can occur at sites where folding structure of the template pauses the RT, allowing a concentration of RNase H cleavages that promote transfers. Transfer can be a simple one-step process, or can proceed by a complex multi-step invasion mechanism. In this latter process, the second RNA template interacts with the growing DNA strand well behind the DNA 3'-terminus. The newly formed RNA-DNA hybrid expands by branch migration and eventually catches the elongating DNA primer 3'-terminus to complete the transfer. Transfers are also promoted by interactions between the two RNA templates, which accelerate transfer by a proximity effect. Other details of the role of strand transfers in reverse transcription and the biochemical features of the transfer reaction are discussed.
1型人类免疫缺陷病毒（HIV-1）和其他逆转录病毒通过逆转录进行复制，在这个过程中，病毒基因组的单链RNA被转化为双链DNA。病毒编码的逆转录酶（RT）通过DNA聚合酶和核糖核酸酶H活性介导逆转录。正链RNA转化为正/负链RNA/DNA杂交体涉及将正在生长的DNA链从基因组RNA上的一个位点转移到另一个位点。这被称为负链强终止DNA转移。随后第二条或正链DNA的合成涉及第二次链转移，其机制与负链转移类似。链转移机制的一个基本特征是利用RT核糖核酸酶H从正在生长的DNA链上移除RNA模板链的片段，释放出一个单链区域以与第二个位点退火。病毒核衣壳蛋白（NC）通过促进这种链交换过程来促进转移。两份RNA基因组，有时并不相同，被共同包装在逆转录病毒的基因组中。逆转录酶的特性使得正在生长的DNA链在逆转录过程中的任何时候偶尔会在这些基因组之间发生转移，产生重组病毒后代。重组促进了病毒的结构多样性，有助于其在宿主免疫和药物治疗中存活。重组链转移可以由模板的断裂引发，或者可以发生在模板的折叠结构使RT暂停的位点，从而导致核糖核酸酶H切割集中，促进转移。转移可以是一个简单的一步过程，也可以通过复杂的多步侵入机制进行。在后一种过程中，第二个RNA模板与正在生长的DNA链在DNA 3'-末端后方很远的位置相互作用。新形成的RNA-DNA杂交体通过分支迁移扩展，最终捕获延伸的DNA引物3'-末端以完成转移。两个RNA模板之间的相互作用也促进了转移，这种相互作用通过邻近效应加速转移。本文还讨论了链转移在逆转录中的作用的其他细节以及转移反应的生化特征。
