Orthopoxvirus genetics

DNA recombination in poxvirus-infected cells is an efficient process which exhibits apparent intramolecular recombination frequencies of 0.006% per base pair of the same order of magnitude as is found in bacteriophage systems. Genetic mapping of ts viral mutants is possible, but the generation of a reliable intergenic recombination map is impractical and of little apparent value in light of our ability to precisely map mutations by marker rescue. Furthermore, since there is a marked variability in the observed intergenic recombiantion frequency measurements and no obvious linear relationship between recombination frequency and the distance between mutations, intergenic recombination frequency measurements are an unsatisfactory tool in poxvirus recombiantion studies. Intragenic recombination frequencies, however, display a linear dependence on the distance separating the mutations up to about 700 bp, so that frequency measurements of this sort can be of value. The difference observed between intergenic and intragenic recombination frequencies suggests that genetic complementation occurs in intergenic crosses at the permissive temperature. The role that this apparent complementation plays in increasing the recombination frequency is unclear. DNA recombination appears to require DNA synthesis in intramolecular viral DNA recombination (Ball 1987); however, in plasmid/plasmid recombination studies, Evans et al. (1988) have shown that while inhibition of viral DNA polymerase decreases the level of recombination, it does not eliminate it. It is not clear if replication is needed to increase the amount of DNA in the infected cells or if replication intermediates serve also as recombination substrates. It is presumed that vaccinia encodes at least some of the enzymes required for viral DNA recombination, although no viral gene product has yet been shown to participate directly in the recombination event. Genetic recombination studies in poxvirus-infected cells are in their infancy. With the development of virus and plasmid recombination systems that are easy to analyze we can anticipate rapid progress in identifying the cellular and viral components involved in the recombination pathway.