Spontaneous deletions and duplications of sequences in the genome of cowpox virus.

Examination of the genomes of 10 white-pock variants of cowpox virus strain Brighton red (CPV-BR) revealed that 9 of them had lost 32 to 38 kilobase pairs (kbp) from their right-hand ends and that the deleted sequences had been replaced by inverted copies of regions from 21 to 50 kbp long from the left-hand end of the genome. These variants thus possess inverted terminal repeats (ITRs) from 21 to 50 kbp long; all are longer than the ITRs of CPV-BR (10 kbp). The 10th variant is a simple deletion mutant that has lost the sequences between 32 and 12 kbp from the right-hand end of the genome. The limits of the inner ends of the observed deletions (between 32 and 38 kbp from the right-hand end of the CPV-BR genome) appear to be defined by the location of the nearest essential gene on the one hand and the location of the gene that encodes "pock redness" on the other. The genomes of the deletion/duplication white-pock variants appear to have been generated either by single crossover recombinational events between two CPV-BR genomes aligned in opposite directions or by the nonreciprocal transfer of genetic information. The sites where such recombination/transfer occurred were sequenced in four variants. In all of them, the sequences adjacent to such sites show no sequence homology or any other unusual structural feature. The analogous sites at the internal ends of the two ITRs of CPV-BR also were sequenced and also show no unusual features. It is likely that the ITRs of CPV-BR and of its white-pock variants, and probably those of other orthopox-virus genomes, arise as a result of nonhomologous recombination or by random nonreciprocal transfer of genetic information.