Torri A F, Englund P T
Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
J Biol Chem. 1995 Feb 24;270(8):3495-7. doi: 10.1074/jbc.270.8.3495.
We previously purified a Crithidia fasciculata mitochondrial DNA polymerase that has unusual properties. Unlike a conventional mitochondrial DNA polymerase gamma, this enzyme is small, non-processive, deficient in 3'-exonuclease activity, and error prone (Torri, A. F., Kunkel, T. A., and Englund, P. T. (1994) J. Biol. Chem. 269, 8165-8171). In all of these characteristics, the enzyme resembles DNA polymerase beta, a nuclear enzyme thought to be involved in DNA repair. We have now cloned and sequenced the gene for this enzyme. The mitochondrial polymerase has significant homology, about 33% identity at the amino acid level, with human DNA polymerase beta. However, sequence analysis of the clone revealed the presence of a cleaved N-terminal presequence, presumably a mitochondrial import signal, which resembles presequences on other C. fasciculata mitochondrial proteins. The polymerase's function may be to repair the many gaps in newly replicated kinetoplast (mitochondrial) DNA minicircles in this parasite. This enzyme is the first example of a mitochondrial DNA polymerase beta.
我们之前纯化了一种具有独特性质的克氏锥虫线粒体DNA聚合酶。与传统的线粒体DNA聚合酶γ不同，这种酶体积小、非持续性、缺乏3'-外切核酸酶活性且易出错（托里，A.F.，昆克尔，T.A.，和英格伦德，P.T.（1994年）《生物化学杂志》269卷，8165 - 8171页）。在所有这些特性方面，该酶类似于DNA聚合酶β，一种被认为参与DNA修复的核酶。我们现已克隆并测序了该酶的基因。这种线粒体聚合酶与人类DNA聚合酶β具有显著的同源性，在氨基酸水平上约有33%的同一性。然而，对该克隆的序列分析揭示了存在一个被切割的N端前序列，推测是一个线粒体导入信号，它类似于其他克氏锥虫线粒体蛋白上的前序列。该聚合酶的功能可能是修复这种寄生虫新复制的动质体（线粒体）DNA微小环中的许多缺口。这种酶是线粒体DNA聚合酶β的首个实例。
