Photochemical addition of the cross-linking reagent 4,5', 8-trimethylpsoralen (trioxaslen) to intracellular and viral simian virus 40 DNA-histone complexes.

We demonstrated here that 4,5', 8-trimethylpsoralen (trioxsalen) is a valuable probe for the structure of SV40 DNA-histone complexes. Trioxsalen readily penetrated intact cells and, in the presence of 340- to 380-nm light, covalently cross-linked DNA preferentially at the sites available for micrococcal nuclease digestion. Histograms of the lengths of the regions of SV40 DNA protected from cross-linking, as visualized by electron microscopy, indicated a repeating pattern of base pairs in DNA from both infected cells and virus particles. The ability of the trioxsalen probe to act in vivo and to map the location of protected regions may provide a powerful tool for analyzing the role of nucleosomes in the structure of the virus particle and in intracellular complexes such as transcription templates and replication intermediates.