Division of Cancer Genetics, Dana-Farber Cancer Institute, 44 Binney Street, Boston, Massachusetts 02115.
Proc Natl Acad Sci U S A. 1983 May;80(10):3025-9. doi: 10.1073/pnas.80.10.3025.
In Chlamydomonas, the maternal inheritance of chloroplast genes correlates with the differential methylation of chloroplast DNA (chlDNA) in females (mt(+)) but not in males (mt(-)). Our previous studies have supported our methylation-restriction model in which the maternal transmission is accounted for by the differential methylation in gametes which protects female but not male chlDNA from degradation during zygote formation. In the mutant me-1 [Bolen, P. L., Grant, D. M., Swinton, D., Boynton, J. E. & Gillham, N. W. (1982) Cell 28, 335-343], chlDNA of vegetative cells of both mating types is heavily methylated even before gametogenesis; nonetheless, maternal inheritance occurs in mutants as in wild type. To investigate the mechanism of maternal inheritance in the me-1 mutant, we have compared restriction fragment patterns after agarose gel electrophoresis of chlDNAs from mutant vegetative cells and gametes with those from wild type, by using a set of 32 restriction enzymes of which 17 were methylation-sensitive in this system. We find that additional methylation occurs during gametogenesis in the mutant female (mt(+)) but not in the corresponding male (mt(-)). Thus, gamete-specific, mating-type-specific methylation occurs in the me-1 mutant as in the wild type, consistent with our methylation-restriction model. In the me-1 mutant, gametic methylation occurs on a background of vegetative cell methylation not present in wild-type cells and irrelevant to the regulation of chloroplast inheritance. Comparison of the me-1 mutation with the mat-1 mutation [Sager, R., Grabowy, C. & Sano, H. (1981) Cell 24, 41-47] provides evidence for the existence of two different chlDNA methylation control systems: mat-1, linked to the mating type locus and regulating the mating-type-specific methylation that correlates with maternal inheritance, and me-1, unlinked to the mating type locus and unrelated to the regulation of maternal inheritance.
在衣藻中，叶绿体基因的母系遗传与雌性（mt(+)）而非雄性（mt(-)）中叶绿体 DNA（chlDNA）的差异甲基化相关。我们之前的研究支持我们的甲基化限制模型，即母系传递是由配子中的差异甲基化引起的，这种差异甲基化保护雌性而非雄性 chlDNA 在合子形成过程中免受降解。在突变体 me-1 中[Bolen, P. L., Grant, D. M., Swinton, D., Boynton, J. E. & Gillham, N. W. (1982) Cell 28, 335-343]，两种交配类型的营养细胞的 chlDNA 在进行配子发生之前就已经高度甲基化；尽管如此，在突变体中仍然会发生母系遗传，就像在野生型中一样。为了研究 me-1 突变体中母系遗传的机制，我们比较了突变体营养细胞和配子中的 chlDNA 在琼脂糖凝胶电泳后的限制片段模式与野生型的模式，使用了一组 32 种限制酶，其中 17 种在该系统中是甲基化敏感的。我们发现突变体雌性（mt(+)）在配子发生过程中会发生额外的甲基化，但对应的雄性（mt(-)）则不会。因此，与野生型一样，me-1 突变体中发生了配子特异性、交配型特异性甲基化，这与我们的甲基化限制模型一致。在 me-1 突变体中，配子甲基化发生在野生型细胞中不存在的营养细胞甲基化背景下，与叶绿体遗传调控无关。将 me-1 突变与 mat-1 突变[Sager, R., Grabowy, C. & Sano, H. (1981) Cell 24, 41-47]进行比较，为存在两种不同的 chlDNA 甲基化控制系统提供了证据：mat-1 与交配型基因座连锁，调节与母系遗传相关的交配型特异性甲基化；而 me-1 与交配型基因座不连锁，与母系遗传的调控无关。
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