Exonuclease III digestion of herpes simplex virus DNA

Circularization and cleavage of guinea pig cytomegalovirus genomes

The mechanisms by which herpesvirus genome ends are fused to form circles after infection and are re-formed by cleavage from concatemeric DNA are unknown. We used the simple structure of guinea pig cytomegalovirus genomes, which have either one repeated DNA sequence at each end or one repeat at one end and no repeat at the other, to study these mechanisms. In circular DNA, two restriction fragments contained fused terminal sequences and had sizes consistent with the presence of single or double terminal repeats. This result implies a simple ligation of genomic ends and shows that circularization does not occur by annealing of single-stranded terminal repeats formed by exonuclease digestion. Cleavage to form the two genome types occurred at two sites, and homologies between these sites identified two potential cis elements that may be necessary for cleavage. One element coincided with the A-rich region of a pac2 sequence and had 9 of 11 bases identical between the two sites. The second element had six bases identical at both sites, in each case 7 bp from the termini. To confirm the presence of cis cleavage elements, a recombinant virus in which foreign sequences displaced the 6- and 11-bp elements 1 kb from the cleavage point was constructed. Cleavage at the disrupted site did not occur. In a second recombinant virus, restoration of 64 bases containing the 6- and 11-bp elements to the disrupted cleavage site restored cleavage. Therefore, cis cleavage elements exist within this 64-base region, and sequence conservation suggests that they are the 6- and 11-bp elements.

Cytomegalovirus: genetic variation of viral genomes

Genomic complexity of human CMV is one of the largest among various DNA viruses. With such genome complexity and widespread distribution, even a minimal frequency of mutation will be enough to create a complicated genetic heterogeneity in this virus. The virus genome is relatively stable during subcultivation in tissue culture. Variants with minor modification in restriction enzyme sites do gradually develop. These variants share substantial restriction fragment pattern homology with the parental virus. Virus DNA has a molecular weight of 150 X 10(6) daltons for a complete genome. The DNA contains 2 covalently linked L and S components which are separated by internal inverted repetitious sequences of both terminal-end sequences. L and S components are oriented invertably with 4 isomeric arrangements. No tandem repeated sequences have been found. Based on DNA restriction pattern analysis, we conclude that the majority of recurrent infections represent reactivation of existing latent viruses; however, reinfections by a new virus strain do occur occasionally. By studying virus strains isolated from the consecutive congenitally infected infants and strains from mothers and their congenitally infected offspring, we furthermore conclude that the latent virus in women is relatively stable genetically. Moreover, the virus strains after being transmitted to the offspring are still genetically similar to that of the mothers. As in vitro, spontaneous minor variations occur after the in vivo residence. In a long period of evolution the accumulation of minor variations might create great diversity with major similarity.