Multiple repetitive DNA sequences in the paracentromeric regions of Arabidopsis thaliana L

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