MULTIPLE REPETITIVE DNA-SEQUENCES IN THE PARACENTROMERIC REGIONS OF ARABIDOPSIS-THALIANA L

Nine repetitive DNA sequences, present in the haploid Arabidopsis thal iana genome in 7-300 copies, were hybridized in site to metaphase and interphase chromosomes. Every sequence was detected on ail five chromo some pairs, but was not evenly dispersed over the genome. Clusters of signals were found in particular regions of the centromeric heterochro matin, and each sequence showed a characteristic distribution pattern. Some sequences hybridized more strongly on different chromosomes, ref lecting chromosome-specific amplification or the presence of homologou s sequences. No hybridization signals could be detected on euchromatic regions. In site hybridization on extended chromatin fibres showed th at the pAL1 repeats are interrupted by another repetitive DNA sequence . A cosmid subclone (74A) contained a (GA)(38) microsatellite motif, a nd hybridization with a (GA) oligonucleotide revealed that most of the hybridization sites of 74A correspond to the distribution of this mic rosatellite motif. The results show that the paracentromeric heterochr omatin of A. thaliana chromosomes is composed not only of the tandemly arranged 180-bp repeat family pAL1/pAtMr, but also of some other repe titive sequences, thus giving a better understanding of the organizati on of sequences at the centromeres of A. thaliana.