Sequence organisation in nuclear DNA from Physarum polycephalum. Interspersion of repetitive and single-copy sequences.

Nuclear DNA from Physarum polycephalum is shown to contain three sequence components by reassociation kinetic analysis; a foldback component consisting of 6% of the DNA, a component with the properties of repetitive sequences comprising 31% of the DNA, and a majority component containing 63% of the DNA which reassociates with the kinetics characteristic of single-copy sequences. The complement of repetitive sequences is comprised of about 80 families of repeated elements, each containing approximately 1800 repeats per family. On average, these sequences are 6.4% richer in guanine and cytosine than total Physarum nuclear DNA. The repetitive sequences within a single family appear not to be identical, since on denaturation and annealing they give rise to collections of heteroduplexes less stable than native DNA. It is calculated that these duplexes are about 10% mismatched on average. Hydroxyapatite binding of DNA fragments of different sizes containing reassociated repeated elements demonstrates that these sequences are interspersed with single-copy sequences in a large portion of the Physarum genome. These observations are confirmed by direct examination of reassociated DNA using the electron microscope. In this manner it is shown that repetitive sequence elements possess a wide spectrum of lengths averaging 590 nucleotide residues, and they are separated by intervening segments of DNA about 930 residues in length.