Satellite DNA IV of Drosophila melanogaster has been isolated in an actinomycin/CsCl gradient and allowed to react with tritium-labeled 4,5′,8-trimethylpsoralen (trioxsalen) in the presence of long wavelength ultraviolet light. Saturation experiments showed a limiting covalent binding of approximately one trioxsalen molecule per three base-pairs for the photochemical reaction. The buoyant density change of satellite IV (ρ = 1·705 g/cm3) after photoreaction was found to be -0·182 (g/cm3)/(trioxsalen/base-pair). To visualize directly the interstrand crosslinks, the photochemically reacted satellite DNA was denatured and spread for electron microscopy. From 85% to 90% of the molecules examined have an appearance of long stretches of tandem loops with few observable double-stranded regions. The histogram of loop sizes in these denatured molecules shows a regular distribution of crosslinkable sites along satellite DNA IV with an interval of approximately 250 base-pairs, suggesting that a long-range periodicity of sequences exists in this satellite DNA. After extensive crosslinking, the observed loops still have sizes that are multiples of 250 base-pairs. These results are discussed in terms of the sequence of satellite DNA IV, the characteristics of the photochemical reaction, and theories of evolution of satellite DNA sequences.
ASJC Scopus subject areas