Simple repetitive (GAA)n loci in the human genome

In order to investigate the organization (and inheritance) of simple tandem (GAA)n repeats in the human genome, different restriction enzymes were employed for DNA digestion followed by separation of the resulting fragments by agarose gel electrophoresis. Frequently cutting enzymes (4 bp recognition sites) revealed highly complex multilocus banding patterns after conventional horizontal submarine gels. Fragments larger than 25 kb, resulting from digestions with rarely cutting enzymes (6 bp recognition sites), were separated by pulsed-field gel electrophoresis (PFGE). Hybridizations were carried out directly in the gel matrix. Nearly all of the enzymes produce at least one predominant signal band after hybridization with (GAA)6. The frequency of (GAA)n stretches was estimated on human chromosome 4 by probing a respective cosmid library. In comparison with other simple di-, tri- and tetranucleotide repeats, (GAA)n stretches appear underrepresented. Hybridization of a fetal human brain cDNA library indicated very few expressed (GAA)n repeats. These data are discussed with particular reference to genomic organization and other simple repetitive trinucleotide stretches.