Tuning Function of Tandemly Repeating Sequences: A Molecular Device for Fast Adaptation

Abstract

Dispersed and tandemly repeating sequences of various types are the most polymorphic components of genomes, especially of eukaryotic genomes. The tandem repeats are also least stable due to frequent changes in the numbers of repeating units in the runs. One may associate this fluid type of genomic polymorphism with individual phenotypic variations and, perhaps, with fast adaptation to a changing environment. A “modulation (or fast adaptation) code” was suggested in 1989 (by the author) that attributed to tandem repeats the function of tuning gene expression by spontaneous changes in the copy numbers of the repeating elements associated with the gene, with subsequent selection. This potentially powerful mechanism of fast adaptation without any changes in the genes themselves was originally illustrated by only a few examples. Since then several researchers have developed similar thoughts, and numerous new supporting examples have appeared in the literature. The changes in copy numbers of tandem repeats are found to influence nearby gene expression. The variable runs of the repeats may, thus, serve as tuners of the gene expression in response to environmental pressures. The same mechanism could serve for differentiation (adaptation of individual cells to a changing cellular environment) and for gradual changes in quantitative traits.