Evidence for intron capture: an unusual path for the evolution of proteins.

Most new genes are thought to evolve from preexisting genes but duplications of entire genes or shuffling of preexisting exons provides only a limited repertoire of new sequences that can be presented to a cell. Only pieces that previously existed can be used in the construction and any further divergence depends on the slow accumulation of mutations. We show here the presence of a small, in-frame intron in a ciliate phosphoglycerate kinase gene and the insertion of an unusually random amino acid sequence at the same position in trypanosome phosphoglycerate kinase. The unusual sequences in trypanosomes were likely to have originally been introns that have been subsequently captured by the protein and have now been incorporated as part of the coding sequence. Via this path a truly unique sequence can be incorporated into an existing protein, leading in time to the evolution of a new, functionally distinct protein. IMAGES:

目前学界普遍认为，绝大多数新基因均由已存在的基因演化而来，但全基因重复或已有外显子（exon）的重组，仅能生成有限范围的新序列，且这类序列仅能由此前已存在的片段构成；任何后续的序列分化，均依赖于突变的缓慢积累。本研究证实，在纤毛虫（ciliate）的磷酸甘油酸激酶（phosphoglycerate kinase）基因中，存在一个小型框内内含子（intron）；而在锥虫（trypanosome）的同源磷酸甘油酸激酶基因的相同位置，则插入了一段异常随机的氨基酸序列。锥虫中的这段异常序列，最初极有可能为内含子，后续被蛋白质捕获并整合至编码序列之中。通过这一途径，全新的独特序列可被整合至已有蛋白质中，最终演化出具备全新功能的独立蛋白质。图像：