Distribution of microsatellites in relation to coding sequences within theArabidopsis thaliana genome

The distribution of repetitive sequences, or microsatellites, formed by eit her one or two base pairs and longer than eight units, has been studied in almost 1 Mb of the sequenced Arabidopsis thaliana genome. Except for those formed by only G and C residues, the repetitions are more abundant in the A rabidopsis genome than can be calculated from its nucleotide composition. T hey are distributed in proportions higher than expected in introns, and in the intergenic regions both proximal and distal to the coding sequences. In exons, only the TC/GA microsatellite stems to be particularly abundant. Th e AT/TA microsatellites produce more length variation between Arabidopsis e cotypes than the A/T repeated sequences. These two classes are more abundan t per kilobase than coding sequences in the Arabidopsis genome. The results indicate not only that the presence of microsatellites is not an effect of random distribution of nucleotides, but that their resolution as molecular markers may be equivalent to the number of genes and also that they do not seem to be systematically linked to specific regulatory sequences proximal to genes. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.