An unusual pattern of genome organization in two Phaseolus plant species

The modes of DNA sequence organization in the nuclear genomes of French bean and green gram are studied. The reassociation kinetics of DNAs of increasing fragment lengths have revealed that approximately 60% of the genomes of French bean and green gram consist of interspersed repeated and single copy sequences. The hyperchromicity and S1-nuclease-resistance of the reassociated long DNA fragments have further confirmed the occurrence of interspersion and have shown the actual proportion of repetitive DNA to be 35-40% in each species. These experiments have also yielded the size of interspersed repeated sequences as 1900-2350 nucleotide pairs. The fragment length of the interspersed single copy DNA sequences is estimated from a curve relating the fraction of DNA fragments binding to hydroxyapatite and the DNA fragment length and is 1300 nucleotides in both plant species. Approximately 35-40% of the single copy sequences are interspersed in this manner in French bean and green gram. Since the lengths of the interspersed repeated sequences are significantly different from those in other plant species, the DNA sequence organization patterns in French bean and green gram are considered to be unusual.