Hancock J M
MRC Clinical Sciences Centre, Royal Postgraduate Medical School, Hammersmith Hospital, London, UK.
J Mol Evol. 1995 Dec;41(6):1038-47. doi: 10.1007/BF00173185.
Simple sequences present in long (> 30 kb) sequences representative of the single-copy genome of five species (Homo sapiens, Caenorhabditis elegans, Saccharomyces cerevisiae, E. coli, and Mycobacterium leprae) have been analyzed. A close relationship was observed between genome size and the overall level of sequence repetition. This suggested that the incorporation of simple sequences had accompanied increases of genome size during evolution. Densities of simple sequence motifs were higher in noncoding regions than in coding regions in eukaryotes but not in eubacteria. All five genomes showed very biased frequency distributions of simple sequence motifs in all species, particularly in eukaryotes where AAA and TTT predominated. Interspecific comparisons showed that noncoding sequences in eukaryotes showed highly significantly similar frequency distributions of simple sequence motifs but this was not true of coding sequences. ANOVA of the frequency distributions of simple sequence motifs indicated strong contributions from motif base composition and repeat unit length, but much of the variation remained unexplained by these parameters. The sequence composition of simple sequences therefore appears to reflect both underlying sequence biases in slippage-like processes and the action of selection. Frequency distributions of simple sequence motifs in coding sequences correlated weakly or not at all with those in noncoding sequences. Selection on coding sequences to eliminate undesirable sequences may therefore have been strong, particularly in the human lineage.
对存在于代表五个物种（智人、秀丽隐杆线虫、酿酒酵母、大肠杆菌和麻风分枝杆菌）单拷贝基因组的长序列（> 30 kb）中的简单序列进行了分析。观察到基因组大小与序列重复的总体水平之间存在密切关系。这表明在进化过程中，简单序列的掺入伴随着基因组大小的增加。在真核生物中，简单序列基序在非编码区的密度高于编码区，但在真细菌中并非如此。所有五个基因组在所有物种中都显示出非常有偏差的简单序列基序频率分布，特别是在以AAA和TTT为主的真核生物中。种间比较表明，真核生物中的非编码序列显示出高度显著相似的简单序列基序频率分布，但编码序列并非如此。对简单序列基序频率分布的方差分析表明，基序碱基组成和重复单元长度有很大贡献，但这些参数仍无法解释许多变异。因此，简单序列的序列组成似乎既反映了类似滑动过程中潜在的序列偏差，也反映了选择的作用。编码序列中简单序列基序的频率分布与非编码序列中的频率分布相关性较弱或根本不相关。因此，对编码序列进行选择以消除不良序列的作用可能很强，特别是在人类谱系中。
