DISTINCT FREQUENCY-DISTRIBUTIONS OF HOMOPOLYMERIC DNA TRACTS IN DIFFERENT GENOMES

The unusual base composition of the genome of the human malaria parasi te Plasmodium falciparum prompted us to systematically investigate the occurrence of homopolymeric DNA tracts in the P. falciparum genome an d, for comparison, in the genomes of Homo sapiens, Saccharomyces cerev isiae, Caenorhabditis elegans, Arabidopsis thaliana, Escherichia coli and Mycobacterium tuberculosis. Comparison of the observed frequencies with the frequencies as expected for random DNA revealed that homopol ymeric (dA:dT) tracts occur well above chance in the eukaryotic genome . In the majority of these genomes, (dA:dT) tract overrepresentation p roved to be an exponential function of the tract length. (dG:dC) tract overrepresentation was absent or less pronounced in both prokaryotic and eukaryotic genomes, On the basis of our results, we propose that h omopolymeric (dA:dT) tracts are expanded via replication slippage. Thi s slippage-mediated expansion does not operate on tracts with lengths below a critical threshold of 7-10 bp.