Masuda Tokiha, Ito Yutaka, Terada Tohru, Shibata Takehiko, Mikawa Tsutomu
Graduate School of Nanobioscience, Yokohama City University, Yokohama 230-0045, USA.
J Biol Chem. 2009 Oct 30;284(44):30230-9. doi: 10.1074/jbc.M109.043810. Epub 2009 Sep 3.
Homologous recombination, which is critical to genetic diversity, depends on homologous pairing (HP). HP is the switch from parental to recombinant base pairs, which requires expansion of inter-base pair spaces. This expansion unavoidably causes untwisting of the parental double-stranded DNA. RecA/Rad51-catalyzed ATP-dependent HP is extensively stimulated in vitro by negative supercoils, which compensates for untwisting. However, in vivo, double-stranded DNA is relaxed by bound proteins and thus is an unfavorable substrate for RecA/Rad51. In contrast, Mhr1, an ATP-independent HP protein required for yeast mitochondrial homologous recombination, catalyzes HP without the net untwisting of double-stranded DNA. Therefore, we questioned whether Mhr1 uses a novel strategy to promote HP. Here, we found that, like RecA, Mhr1 induced the extension of bound single-stranded DNA. In addition, this structure was induced by all evolutionarily and structurally distinct HP proteins so far tested, including bacterial RecO, viral RecT, and human Rad51. Thus, HP includes the common non-canonical DNA structure and uses a common core mechanism, independent of the species of HP proteins. We discuss the significance of multiple types of HP proteins.
同源重组对遗传多样性至关重要，它依赖于同源配对（HP）。HP是从亲本碱基对到重组碱基对的转变，这需要碱基对间空间的扩展。这种扩展不可避免地会导致亲本双链DNA的解旋。RecA/Rad51催化的ATP依赖性HP在体外受到负超螺旋的广泛刺激，负超螺旋可补偿解旋。然而，在体内，双链DNA因结合蛋白而松弛，因此是RecA/Rad51的不利底物。相比之下，Mhr1是酵母线粒体同源重组所需的一种不依赖ATP的HP蛋白，它催化HP时双链DNA不会发生净解旋。因此，我们质疑Mhr1是否采用了一种新策略来促进HP。在这里，我们发现，与RecA一样，Mhr1诱导结合的单链DNA延伸。此外，到目前为止测试的所有进化和结构不同的HP蛋白，包括细菌RecO、病毒RecT和人类Rad51，都能诱导这种结构。因此，HP包括常见的非经典DNA结构，并使用共同的核心机制，与HP蛋白的种类无关。我们讨论了多种类型HP蛋白的意义。
