Characteristic enrichment of DNA repeats in different genomes

Using computer programs developed for this purpose, we searched for various repeated sequences including inverted,directtandem,andhomopurine-homopyrimidinemir- ror repeats in various prokaryotes, eukaryotes, and an archae- bacterium. Comparison of observed frequencies with expecta- tions revealed that in bacterial genomes and organelles the frequency of different repeats is either random or enriched for inverted andyor direct tandem repeats. By contrast, in all eukaryoticgenomesstudied,weobservedanoverrepresentation of all repeats, especially homopurine-homopyrimidine mirror repeats. Analysis of the genomic distribution of all abundant repeats showed that they are virtually excluded from coding sequences. Unexpectedly, the frequencies of abundant repeats normalized for their expectations were almost perfect exponen- tialfunctionsoftheirsize,andforagivenrepeatthisfunctionwas indistinguishable between different genomes.