Adaptive evolution of transcription factor binding sites.
BACKGROUND
The regulation of a gene depends on the binding of transcription factors to specific sites located in the regulatory region of the gene. The generation of these binding sites and of cooperativity between them are essential building blocks in the evolution of complex regulatory networks. We study a theoretical model for the sequence evolution of binding sites by point mutations. The approach is based on biophysical models for the binding of transcription factors to DNA. Hence we derive empirically grounded fitness landscapes, which enter a population genetics model including mutations, genetic drift, and selection.


RESULTS
We show that the selection for factor binding generically leads to specific correlations between nucleotide frequencies at different positions of a binding site. We demonstrate the possibility of rapid adaptive evolution generating a new binding site for a given transcription factor by point mutations. The evolutionary time required is estimated in terms of the neutral (background) mutation rate, the selection coefficient, and the effective population size.


CONCLUSIONS
The efficiency of binding site formation is seen to depend on two joint conditions: the binding site motif must be short enough and the promoter region must be long enough. These constraints on promoter architecture are indeed seen in eukaryotic systems. Furthermore, we analyse the adaptive evolution of genetic switches and of signal integration through binding cooperativity between different sites. Experimental tests of this picture involving the statistics of polymorphisms and phylogenies of sites are discussed.
Berg
Johannes
J
Institute for Theoretical Physics, University of Cologne, 50937 Cologne, Germany. berg@thp.uni-koeln.de <berg@thp.uni-koeln.de>
Willmann
Stana
S
Lässig
Michael
M
D000818:Animals; D001665:Binding Sites / Q000235:genetics; D004247:DNA / Q000235:genetics; D004269:DNA, Bacterial / Q000235:genetics; D004330:Drosophila / Q000235:genetics; D004331:Drosophila melanogaster / Q000235:genetics; D048168:Escherichia coli K12 / Q000235:genetics; D019143:Evolution, Molecular; D040961:Genetic Drift; D005828:Genetics, Population / Q000379:methods / Q000706:statistics & numerical data; D008957:Models, Genetic; D017354:Point Mutation / Q000235:genetics; D012641:Selection, Genetic; D014157:Transcription Factors / Q000235:genetics*
D004269:DNA, Bacterial; D014157:Transcription Factors; D004247:DNA