After several serial passages at low multiplicities of infection in primary human foetal glial cells at 37 °C, the DNA of prototype (MAD-1) JC virus and that of MAD-2 and MAD-3 are typically heterogeneous in size, but DNAs of MAD-4 and MAD-6 are relatively homogeneous. A similar dichotomy was observed in the DNAs of six isolates propagated more recently in glial cultures at 39 °C under similar conditions of brief passage at low multiplicities of infection: the DNAs of two (MAD-9 and -10) were heterogeneous, but the DNAs of four others (MAD-8, -11, -12 and -14) were homogeneous. Therefore, the propensity of the viral genome to sustain deletions was an intrinsic property of each isolate. However, actual induction and maintenance of the presumably defective DNAs was influenced by the relative proportions of permissive spongioblasts and semi-permissive astrocytes in the glial cultures and by the multiplicity of infection. Deletions in MAD-1 DNA were confined to the presumptive early region and spanned the HI cleavage site (map position 0.505). The heterogeneity was more complex in the DNAs of MAD-2 and MAD-3, but again most of the deletions, which ranged up to 12% of full-length DNA, spanned the HI site. We propose that the differential susceptibility to deletion among isolates is a consequence of natural genetic variation in JC virus.


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