The amplification of oligonucleotide themes in the evolution of the myc protooncogene family

The evolutionary past of intragenic repeats in protein-coding exons of c-, N-, L-, and s-myc-protooncogene subfamilies was elucidated. Apparently these genes evolved by succession of distinct unit events rather than by a steady flow of random point mutations. An evolutionary event probably involved a duplication of the whole gene, which was followed by amplification of progressively shorter oligonucleotide themes and motifs. The repeats were either joined in tandem or one of the copies was transposed and integrated elsewhere within the same exon. In some instances multiple fragments of an amplified theme were integrated at several sites. Direct repeats were found to prevail over inverted ones. By reconstructing the fate of repeats in the course of evolution of vertebrates, the origins of some functional domains could be traced to the initial amplification event. For example, an N-myc-specific domain was created by tandem duplication of a single-copy theme of L-myc exon; at the time of divergence of the c-myc and N-myc, the tandem duplex underwent a new round of duplication followed by transposition of the new copy, thus accounting for the formation of a new domain specific for c-myc. The model proposed here may be regarded as a molecular-level equivalent of the theory of punctuated equilibria.