SV40 DNA Molecules Which Contain Host Cell DNA
Closed-circular SV40 DNA molecules, in which parts of the viral genome are deleted and replaced by covalently-linked cell DNA (called substituted SV40 DNA) are produced in monkey BS-C-1 cells infected under certain conditions (1,2). The proportions of such substituted molecules in the yield depend upon the multiplicity of infection and the passage history of the inoculum (2). Although substituted SV40 DNA molecules are defective with respect to plaque formation, they nevertheless replicate in cells infected at high multiplicity (3), presumably by complementation with non-substituted virus. The proportion and types of host sequences in the substituted viral genome have been found to vary in different populations. In one population examined, the host sequences comprised as much as 60 percent of the substituted viral genome (4). By hybridization analysis, these host sequences were found to be both of the reiterated and non-reiterated type (4,5). In other populations, the host sequences were almost exclusively of the non-reiterated type (4). We have compared, by homology tests, the host sequences in three independently-derived populations of substituted SV40 DNA molecules (4). Populations A and B were derived by independent sets of high-multiplicity serial passages, starting from the same plaque purified virus stock; population C arose during the serial passage of non-plaque purified virus. No detectable cross-homology was found between the host sequences of populations A and B; on the other hand, population C contained a significant fraction of host sequences which was homologous to those in both the A and B populations. Hence, the host sequences in substituted SV40 cannot represent a random sample of the total sequences present in the mammalian genome. The number of different families of host sequences that can be incorporated into the SV40 genome is clearly limited.
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