Unstable Tandem Repeats in Promoters Confer Transcriptional Evolvability

Genotype, Phenotype, and Variation Phenotypic evolution results from changes in coding sequences, as well as in regulatory regions of the genome. Vinces et al. (p. 1213) uncover an abundant source of gene expression variability in the promoters of Saccharomyces cerevisiae genes. Up to 25% of yeast promoter regions contain a highly variable tandem repeat sequence, which effect gene expression and local chromatin structure. The intrinsic instability of these repeats generates variability in gene expression levels and provides a mechanism for rapid evolution of phenotypic traits. Unstable repetitive DNA in the control regions of yeast genes modulates the evolution of chromatin structure and gene expression. Relative to most regions of the genome, tandemly repeated DNA sequences display a greater propensity to mutate. A search for tandem repeats in the Saccharomyces cerevisiae genome revealed that the nucleosome-free region directly upstream of genes (the promoter region) is enriched in repeats. As many as 25% of all gene promoters contain tandem repeat sequences. Genes driven by these repeat-containing promoters show significantly higher rates of transcriptional divergence. Variations in repeat length result in changes in expression and local nucleosome positioning. Tandem repeats are variable elements in promoters that may facilitate evolutionary tuning of gene expression by affecting local chromatin structure.

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