Department of Genetics, University of Leicester, Leicester, UK.
DNA Repair (Amst). 2011 Feb 7;10(2):126-37. doi: 10.1016/j.dnarep.2010.09.024. Epub 2010 Nov 1.
Exo1 is a member of the Rad2 protein family and possesses both 5'-3' exonuclease and 5' flap endonuclease activities. In addition to performing a variety of functions during mitotic growth, Exo1 is also important for the production of crossovers during meiosis. However, its precise molecular role has remained ambiguous and several models have been proposed to account for the crossover deficit observed in its absence. Here, we present physical evidence that the nuclease activity of Exo1 is essential for normal 5'-3' resection at the Spo11-dependent HIS4 hotspot in otherwise wild-type cells. This same activity was also required for normal levels of gene conversion at the locus. However, gene conversions were frequently observed at a distance beyond that at which resection was readily detectable arguing that it is not the extent of the initial DNA end resection that limits heteroduplex formation. In addition to these nuclease-dependent functions, we found that an exo1-D173A mutant defective in nuclease activity is able to maintain crossing-over at wild-type levels in a number of genetic intervals, suggesting that Exo1 also plays a nuclease-independent role in crossover promotion.
Exo1 是 Rad2 蛋白家族的一员，具有 5'-3' 外切核酸酶和 5' 发夹内切核酸酶活性。除了在有丝分裂生长过程中发挥多种功能外，Exo1 在减数分裂过程中产生交叉也很重要。然而，其确切的分子作用仍然不清楚，已经提出了几种模型来解释其缺失时观察到的交叉缺陷。在这里，我们提供了物理证据，证明 Exo1 的核酸酶活性对于在 otherwise wild-type 细胞中 Spo11 依赖性 HIS4 热点处的正常 5'-3' 切除至关重要。该活性对于该基因座的正常基因转换水平也是必需的。然而，基因转换经常在远远超过易于检测到的切除范围的距离处观察到，这表明不是初始 DNA 末端切除的程度限制异源双链体的形成。除了这些依赖核酸酶的功能外，我们发现 Exo1-D173A 突变体（一种在核酸酶活性上有缺陷的突变体）能够在多个遗传间隔中维持与野生型水平相当的交叉频率，这表明 Exo1 在促进交叉方面也发挥了非依赖核酸酶的作用。
