编码序列和非编码序列的3-tuple分布特征

非编码序列，特别是内含子的起源，是一个重要的悬而未决的问题。首先通过计算模式生物的编码序列和非编码序列的不同阅读框中3-tupie的频率分布，发现编码区中不同阅读框具有十分不同的3-tuple分布，而在非编码区中，不同阅读框的3-tuple分布几乎相等，并且这一性质不具有物种依赖性。为了描述分布差异的程度，引进夏量一对称相对熵，并通过比较原核生物和真核生物，发现无论是编码区还是非编码区，原核生物都具有比真核生物更高的SRE值。进一步研究表明，某一生物的SRE值与该生物全基因组中编码区所占的百分比存在一定的相关性（相关系数为0．86）。计算机模拟进化实验发现，2％的突变就足以使典型的嗯核生物编码区高SRE值变为真核生物内含子区特有的低SRE值。比对数据库中已经注释的内含子和编码区序列，证明确实有一部分与编码区具有很高同源性的内含子序列。实验表明，至少部分真核生物的内含子可能起源于编码序列，同时也说明SRE可能被用于研究物种基因组序列的进化。

Features of Coding and Noncoding Sequences Based on 3-tuple Distributions

The origin of non-coding sequences,especially introns,is an outstanding issue that has been receiving continuous debate for the last two decades.In the current work we use a mathematical model to characterize DNA sequences and find that the 3-tuple distributions in different reading frames of a given coding sequence differ sharply from each other,while they are almost identical to each other in introns or other non-coding sequences.SREs(Symmetric relative entropies) decrease progressively from coding sequences of primitive prokaryotes to those of advanced eukaryotes and from non-coding sequences of low eukaryotes to those of high eukaryotes with a correlation coefficient of 0.86.In silico evolution experiments show that SREs typical of higher eukaryotic introns can be achieved from prokaryotic coding sequences as the mutation ratio reaches 2/100.The fact that (a total of 25 introns) from all three different genomes S.pombe,C.elegans and H.sapiens searched are found to share high sequence identity with coding regions indicates that at least some introns may have come directly from CDS(coding sequences).We suggest that SREs may be a useful feature for evolutionary study.