Diener T O
Center for Agricultural Biotechnology, University of Maryland, College Park, 20742.
Proc Natl Acad Sci U S A. 1989 Dec;86(23):9370-4. doi: 10.1073/pnas.86.23.9370.
The demonstration of enzymatic capabilities of certain RNAs, in addition to their well-known template properties, has led to the recognition that RNAs are the only biological macromolecules that can function both as genotype and phenotype, hence raising the possibility of Darwinian selection and precellular evolution at the RNA level in the absence of DNA or protein. Recent models of such precellular RNA systems are patterned after the properties of intron-derived ribozymes. On the basis of a phylogenetic analysis and known properties of certain small plant pathogenic RNAs (viroids and viroid-like satellite RNAs), I suggest that these plant RNAs are more plausible candidates than introns as "living fossils" of a precellular RNA world. Their small size and circularity would have enhanced probability of their survival in error-prone, primitive self-replicating RNA systems and assured complete replication without the need for initiation or termination signals. All of these RNAs possess efficient mechanisms for the precise cleavage of monomers from oligomeric replication intermediates. Some (most viroids) require a host factor, but others (viroid-like satellite RNAs and one viroid) function as self-cleaving RNA enzymes far smaller and simpler than those derived from introns. The question is raised whether introns could have evolved from viroids or viroid-like satellite RNAs rather than vice versa, as has been widely speculated.
某些RNA除了具有众所周知的模板特性外，还展现出酶促能力，这使得人们认识到RNA是唯一既能作为基因型又能作为表型发挥作用的生物大分子，因此增加了在没有DNA或蛋白质的情况下，RNA水平上达尔文选择和细胞前进化的可能性。这种细胞前RNA系统的最新模型是根据内含子衍生核酶的特性构建的。基于系统发育分析以及某些小型植物致病RNA（类病毒和类病毒样卫星RNA）的已知特性，我认为这些植物RNA比内含子更有可能是细胞前RNA世界的“活化石”。它们的小尺寸和环状结构会增加其在容易出错的原始自我复制RNA系统中存活的概率，并确保无需起始或终止信号就能完全复制。所有这些RNA都拥有从寡聚复制中间体精确切割单体的有效机制。有些（大多数类病毒）需要宿主因子，但其他一些（类病毒样卫星RNA和一种类病毒）作为自我切割RNA酶发挥作用，比从内含子衍生的酶要小得多且简单得多。有人提出了一个问题，即内含子是否可能是从类病毒或类病毒样卫星RNA进化而来，而不是像人们广泛推测的那样相反。
