Instability and reiteration of DNA sequences within the vaccinia virus genome

The sequence arrangement within the nontranscribed portion of the inverted terminal repetition of the vaccinia virus genome exists in quasi-stable and unstable forms that are not distinguishable on the basis of viral infectivity. The unstable forms, which composed about 20% of a serially passaged stock of virus, were recognized by terminal heterogeneity on restriction endonuclease analysis. Instead of a single terminal fragment from each end of the genome, an array of eight or more fragments differing in size by 1650-base-pair increments was detected. This feature was not eliminated by repeated plaque purification, indicating that the population of DNA molecules with various numbers of reiterations can rapidly evolve from the DNA of a single virus particle. However, at each successive round of plaque purification, about 20% of the unstable isolates revert back to the more stable form. Stable forms are characterized by the presence of a set of 13-17 tandem 70-base-pair repeats on each side of a 435-base-pair intervening sequence near both ends of the genome. In contrast, the unstable forms possess sets of tandem repeats and intervening sequences that alternate many times in series. The transition between the two genomic forms and the evolution of the unstable form appear to be mediated by recombinational events.