Herridge Rowan P, Day Robert C, Macknight Richard C
Department of Biochemistry, University of Otago, Dunedin, 9054, New Zealand.
Plant Mol Biol. 2014 Sep;86(1-2):69-84. doi: 10.1007/s11103-014-0213-x. Epub 2014 Jun 20.
The MINICHROMOSOME MAINTENANCE 2-7 (MCM2-7) complex, a ring-shaped heterohexamer, unwinds the DNA double helix ahead of the other replication machinery. Although there is evidence that individual components might have other roles, the essential nature of the MCM2-7 complex in DNA replication has made it difficult to uncover these. Here, we present a detailed analysis of Arabidopsis thaliana mcm2-7 mutants and reveal phenotypic differences. The MCM2-7 genes are coordinately expressed during development, although MCM7 is expressed at a higher level in the egg cell. Consistent with a role in the egg cell, heterozygous mcm7 mutants resulted in frequent ovule abortion, a phenotype that does not occur in other mcm mutants. All mutants showed a maternal effect, whereby seeds inheriting a maternal mutant allele occasionally aborted later in seed development with defects in embryo patterning, endosperm nuclear size, and cellularization, a phenotype that is variable between subunit mutants. We provide evidence that this maternal effect is due to the necessity of a maternal store of MCM protein in the central cell that is sufficient for maintaining seed viability and size in the absence of de novo MCM transcription. Reducing MCM levels using endosperm-specific RNAi constructs resulted in the up-regulation of DNA repair transcripts, consistent with the current hypothesis that excess MCM2-7 complexes are loaded during G1 phase, and are required during S phase to overcome replicative stress or DNA damage. Overall, this study demonstrates the importance of the MCM2-7 subunits during seed development and suggests that there are functional differences between the subunits.
微小染色体维持蛋白2 - 7（MCM2 - 7）复合体是一种环状异源六聚体，它在其他复制机制之前解开DNA双螺旋。尽管有证据表明单个组件可能具有其他作用，但MCM2 - 7复合体在DNA复制中的本质作用使得难以揭示这些作用。在这里，我们对拟南芥mcm2 - 7突变体进行了详细分析，并揭示了表型差异。MCM2 - 7基因在发育过程中协同表达，尽管MCM7在卵细胞中的表达水平较高。与在卵细胞中的作用一致，杂合mcm7突变体导致频繁的胚珠败育，这一表型在其他mcm突变体中未出现。所有突变体都表现出母本效应，即继承母本突变等位基因的种子偶尔会在种子发育后期败育，伴有胚胎模式形成、胚乳核大小和细胞化方面的缺陷，这一表型在亚基突变体之间存在差异。我们提供的证据表明，这种母本效应是由于中央细胞中母本储存的MCM蛋白的必要性，该蛋白在没有从头MCM转录的情况下足以维持种子活力和大小。使用胚乳特异性RNAi构建体降低MCM水平导致DNA修复转录本上调，这与当前的假设一致，即过量的MCM2 - 7复合体在G1期加载，并在S期需要以克服复制应激或DNA损伤。总体而言，这项研究证明了MCM2 - 7亚基在种子发育过程中的重要性，并表明亚基之间存在功能差异。
