Shiba Kiyotaka, Takahashi Yuki, Noda Tetsuo
Department of Protein Engineering, Cancer Institute, Japanese Foundation for Cancer Research, Toshima, Tokyo 170-8455, Japan.
J Mol Biol. 2002 Jul 19;320(4):833-40. doi: 10.1016/s0022-2836(02)00567-3.
Two different views have been proposed for origins of genes (or proteins). One is that primordial genes evolved from random sequences. This view underlies the concept of modern in vitro evolution experiments that functional molecules (even proteins) evolved from random sequence-libraries. On the contrary, the second view reminds that "random sequences" would be an unusual state in which to find RNA or DNA, because it is their inherent nature to yield periodic structures during the course of semi-conservative replication. In this second view, the periodicity of DNA (or RNA) is responsible for emergence of primordial genes. Although recent reports on the variety of periodicities present in proteins, genes and genomes are consistent with the second view, it has yet to be experimentally tested. We assessed the significance of periodicities of DNA in the origin of genes by constructing such periodic DNAs. The results showed that periodic DNA produced ordered proteins at very high rates, which is in contrast to the fact that proteins with random sequences lack secondary structures. We concluded that periodicity played a pivotal role in the origin of many genes. The observation should pave the way for new experimental evolution systems for proteins.
关于基因（或蛋白质）的起源，已经提出了两种不同的观点。一种观点认为原始基因是从随机序列进化而来的。这种观点构成了现代体外进化实验概念的基础，即功能分子（甚至蛋白质）是从随机序列文库中进化而来的。相反，第二种观点提醒人们，“随机序列”对于RNA或DNA来说是一种不寻常的状态，因为在半保留复制过程中产生周期性结构是它们的固有特性。在第二种观点中，DNA（或RNA）的周期性是原始基因出现的原因。尽管最近关于蛋白质、基因和基因组中存在的各种周期性的报道与第二种观点一致，但尚未经过实验验证。我们通过构建这种周期性DNA来评估DNA周期性在基因起源中的重要性。结果表明，周期性DNA以非常高的速率产生有序蛋白质，这与随机序列的蛋白质缺乏二级结构的事实形成对比。我们得出结论，周期性在许多基因的起源中起着关键作用。这一观察结果应该为蛋白质的新实验进化系统铺平道路。
