Characterization of Rapidly Reassociating Components of the Dna of Some Higher Plants

The relative number of copies of pumpkin DNA sequences which are complementary to high molecular weight ribosomal RNA (HMW rRNA), 5S and 4S RNA was found, by DNA-RNA hybridization experiments, to be approximately 1:1.2:2.5. At saturation, the levels of hybridization obtained with these RNAs were 2.4%, 0.029% and 0.044%, respectively. Based upon a haploid genomic DNA content of 4.9 x 10^' daltons; this corresponds to 3200 , 3700 and 8100 copies of genes for these RNAs. These gene sequences are referred to as HMW rDNA, 5S DNA and 4S DNA. When banded in CsCI, pumpkin DNA possesses a main band and a satellite component, the buoyant density of which are 1.695 g/cc and 1.708 g/cc, respectively. It was found that HMW rDNA and 5S DNA are contained in the satellite component, whereas 4S DNA is located in both satellite and main band DNA. Eukaryote DNA contains many families of repetitive sequences in addition to HMW rDNA, 5S and 4S DNA. The repetitive sequences may be separated from unique sequences by renaturing the single stranded DNAs under limiting conditions such that only the most repetitive sequences reanneal. The DNA of several plant species which renatured at Cot = 0.14 was termed low Cot DNA and was separated from single stranded DNA by hydroxyapatite chromatography. The proportions of the DNA of pumpkin, Chinese cabbage, pinto bean and tobacco which proved to be low Cot DNA were 23, 21, 18 and 11% respectively.