ANALYSIS OF THE CHROMATIN DOMAIN ORGANIZATION AROUND THE PLASTOCYANINGENE REVEALS AN MAR-SPECIFIC SEQUENCE ELEMENT IN ARABIDOPSIS-THALIANA

The Arabidopsis thaliana genome is currently being sequenced, eventual ly leading towards the unravelling of all potential genes. We wanted t o gain more insight into the way this genome might be organized at the ultrastructural level. To this extent we identified matrix attachment regions demarking potential chromatin domains, in a 16 kb region arou nd the plastocyanin gene. The region was cloned and sequenced revealin g six genes in addition to the plastocyanin gene. Using an heterologou s in vitro nuclear matrix binding assay, to search for evolutionary co nserved matrix attachment regions (MARs), we identified three such MAR s. These three MARs divide the region into two small chromatin domains of 5 kb, each containing two genes. Comparison of the sequence of the three MARs revealed a degenerated 21 bp sequence that is shared betwe en these MARs and that is not found elsewhere in the region. A similar sequence element is also present in four other MARs of Arabidopsis. T herefore, this sequence may constitute a landmark for the position of MARs in the genome of this plant. In a genomic sequence database of Ar abidopsis the 21 bp element is found approxiamally once every 10 kb. T he compactness of the Arabidopsis genome could account for the high in cidence of MARs and MRSs we observed.