After six to eight serial undiluted passages of mouse hepatitis virus (JHM strain) in DBT cell culture, a decrease in the yield of infectious virus occurred, and with further passages fluctuating yields of infectious virus were observed. The serially passaged virus interfered with the multiplication of the standard JHM virus, but not with vesicular stomatitis virus. After sucrose equilibrium centrifugation of high passage virus, a single peak contained both infectious virus and interfering activity. This virus population resembled the original JHM virus in its structural proteins, but it contained an increased proportion of a protein with a molecular weight of 65 × 103. Genomic RNA from standard JHM virus contained a single species of RNA with a molecular weight of 5.4 × 106. After five undiluted passages, however, the virion population contained two RNA species with molecular weights of 5.4 × 106 and 5.2 × 106. RNase T1 resistant oligonucleotide finger-printing of these RNAs showed that the lower molecular weight RNA had lost several oligonucleotide spots that were present in the genomic RNA of the standard JHM virus. After several serial diluted passages of passage 10 virus, a single virus population was obtained which again had only standard virus RNA with a molecular weight of 5.4 × 106 and lacked interfering activity. These results indicated that defective interfering particles were generated by serial undiluted passages of JHM virus.
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