MICROSATELLITE SPREADING IN THE HUMAN GENOME - EVOLUTIONARY MECHANISMS AND STRUCTURAL IMPLICATIONS

Microsatellites are tandem repeat sequences abundant in the genomes of higher eukaryotes and hitherto considered as ''junk DNA,'' Analysis o f a human genome representative data base (2.81 Mb) reveals a distinct Juxtaposition of A-rich microsatellites and retroposons and suggests their coevolution, The analysis implies that most microsatellites were generated by a 3'-extension of retrotranscripts, similar to mRNA poly adenylylation, and that they serve in turn as ''retroposition navigato rs,'' directing the retroposons via homology-driven integration into d efined sites. Thus, they became instrumental in the preservation and e xtension of primordial genomic patterns, A role is assigned to these r eiterating A-rich loci in the higher-order organization of the chromat in, The disease-associated triplet repeats are mostly found in coding regions and do not show an association with retroposons, constituting a unique set within the family of microsatellite sequences.