Two discrete classes of particles appear following the infection of primary African green monkey kidney cells with high multiplicities of an SV40-like isolate from a patient with progressive multifocal leukoencephalopathy (PML). The DNA purified from the nondefective "heavy" particles was very similar to standard SV40 DNA. "Light" viral DNA was circular, supercoiled, 80% the physical size of SV40, and did not contain the EcoRI restriction endonuclease site. The kinetics of "light" viral DNA reassociation indicated a physical complexity equivalent to 40-50% of SV40 DNA; no host sequences were detected in "light" viral DNA. Digestion of "light" viral DNA with the Hemophilus influenzae restriction endonuclease resulted in 5 cleavage products none of which corresponded to the 11 fragments obtained with SV40 DNA.
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