Selker E U, Stevens J N
Proc Natl Acad Sci U S A. 1985 Dec;82(23):8114-8. doi: 10.1073/pnas.82.23.8114.
We describe two unusual 5S RNA regions from Neurospora crassa that are tightly linked. Sequence analysis suggests that these genes or pseudogenes, which we designate zeta (zeta) and eta (eta), arose by a 794-base-pair tandem duplication followed by hundreds of exclusively cytosine to thymine mutations. The duplication was most likely generated by nonhomologous recombination involving a DNA segment having a striking purine-pyrimidine strand asymmetry. Restriction analysis of genomic DNA from tissue grown in the presence or absence of 5-azacytidine indicates that many, and perhaps all, cytosines in the duplicated region are methylated in most cells. This is in contrast to the situation typically observed in eukaryotes, where 5-methylcytosine is found only at positions one or two nucleotides preceding guanine residues. No DNA methylation was detected in the unique DNA flanking the zeta-eta duplication. Thus the "signal" for methylation may be the duplication itself. The numerous transition mutations in this region probably occurred by deamination of 5-methylcytosines. Our results suggest that DNA methylation can have important evolutionary consequences in eukaryotes.
我们描述了来自粗糙脉孢菌的两个紧密连锁的不寻常的5S RNA区域。序列分析表明，这些基因或假基因，我们将其命名为ζ（zeta）和η（eta），是由一个794个碱基对的串联重复产生的，随后发生了数百个胞嘧啶到胸腺嘧啶的突变。这种重复很可能是由涉及具有显著嘌呤-嘧啶链不对称性的DNA片段的非同源重组产生的。对在有或没有5-氮杂胞苷的情况下生长的组织的基因组DNA进行限制性分析表明，在大多数细胞中，重复区域中的许多（也许是所有）胞嘧啶都被甲基化了。这与真核生物中通常观察到的情况相反，在真核生物中，5-甲基胞嘧啶仅在鸟嘌呤残基前一两个核苷酸的位置被发现。在ζ-η重复侧翼的独特DNA中未检测到DNA甲基化。因此，甲基化的“信号”可能就是重复本身。该区域中众多的转换突变可能是由5-甲基胞嘧啶的脱氨作用发生的。我们的结果表明，DNA甲基化在真核生物中可能具有重要的进化后果。
