CccDNA

cccDNA即共价闭合环状DNA (covalently closed circular DNA)是一种有三级结构的双链环状DNA分子，最早是在噬菌体中发现的，而在自然界中被DNA病毒（如多瘤病毒科）感染的其它细胞中也有发现。^[1]^[2]

产生

cccDNA在一些病毒侵染宿主细胞核时产生并在随后永久的存在于核内，其前体是一种线型的双链DNA，通过DNA连接酶作用闭环而形成cccDNA。大部分情况下这些病毒的DNA只有在这样的状态下才能进行转录。cccDNA通常被认为是游离的DNA或是“微小基因组” (minigenome)，但质粒不一定是cccDNA。

存在形式

cccDNA的存在形式在逆转录病毒（或者说单链RNA逆转录病毒，如正反转录病毒亚科）和拟逆转录病毒（即双链DNA逆转录病毒）的侵染过程中是相同的。然而前者DNA立刻整合到宿主基因组中，后者则是游离在核质中。

实例

cccDNA是花椰菜病毒科和肝病毒科（包括乙肝病毒 HBV）的典型特征。其中乙肝病毒(HBV)的cccDNA的前体是与蛋白质外壳相连的“松弛环状DNA”（rcDNA 或 relaxed circular DNA），其本质是含有可配对的黏性末端的线型DNA（因碱基互补配对而成环状）。^[3] 被乙肝病毒感染之后，通过治疗可以使cccDNA不易被活化，因此cccDNA的活性是乙肝的治疗中的一个指标。^[4]

结构

参见　DNA超螺旋

参见

资料来源

^ Mosevitskaia, T.V., E.B. Pavel'chuk, and N.V. Tomilin. Substrate of a UV-induced repair system providing for W-reactivation of lambda phage. Genetika. 1976, 12 (8): 131–138. PMID 1001892.
^ Kunisada, T. and H. Yamagishi. Sequence repetition and genomic distribution of small polydisperse circular DNA purified from HeLa cells. Gene. November 1984, 31 (1–3): 213–223. PMID 6098526. doi:10.1016/0378-1119(84)90212-9.
^ Guo H., D. Jiang, T. Zhou, A. Cuconati, T.M. Block, and J.T. Guo. Characterization of the intracellular deproteinized relaxed circular DNA of hepatitis B virus: an intermediate of covalently closed circular DNA formation. J Virol. November 2007, 81 (22): 12472–12484. PMC 2169032  . PMID 17804499. doi:10.1128/JVI.01123-07.
^ Bourne, E.J.; Dienstag, J.L.; Lopez, V.A.; et al. Quantitative analysis of HBV cccDNA from clinical specimens: correlation with clinical and virological response during antiviral therapy. Journal of Viral Hepatitis. January 2007, 14 (1): 56–63. doi:10.1111/j.1365-2893.2006.00775.x.

[Mosevitskaia1976-1] Mosevitskaia, T.V., E.B. Pavel'chuk, and N.V. Tomilin. Substrate of a UV-induced repair system providing for W-reactivation of lambda phage. Genetika. 1976, 12 (8): 131–138. PMID 1001892.

[Kunisada1984-2] Kunisada, T. and H. Yamagishi. Sequence repetition and genomic distribution of small polydisperse circular DNA purified from HeLa cells. Gene. November 1984, 31 (1–3): 213–223. PMID 6098526. doi:10.1016/0378-1119(84)90212-9.

[Guo2007-3] Guo H., D. Jiang, T. Zhou, A. Cuconati, T.M. Block, and J.T. Guo. Characterization of the intracellular deproteinized relaxed circular DNA of hepatitis B virus: an intermediate of covalently closed circular DNA formation. J Virol. November 2007, 81 (22): 12472–12484. PMC 2169032  . PMID 17804499. doi:10.1128/JVI.01123-07.

[Bourne2007-4] Bourne, E.J.; Dienstag, J.L.; Lopez, V.A.; et al. Quantitative analysis of HBV cccDNA from clinical specimens: correlation with clinical and virological response during antiviral therapy. Journal of Viral Hepatitis. January 2007, 14 (1): 56–63. doi:10.1111/j.1365-2893.2006.00775.x.

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