Recombination between a defective retrovirus and homologous sequences in host DNA: reversion by patch repair.

The genomes of mammalian species contain multiple copies of sequences homologous to exogenous retroviruses. When a mutant retrovirus carrying a lethal deletion in an essential viral gene was introduced into mammalian cells, revertant viruses appeared and spread throughout the culture. Analysis of one such revertant showed that the mutation had been repaired by homologous recombination with endogenous sequences. Our results suggest that defective retroviruses can draw upon the genetic complement of the host cell to repair lesions in viral genes.