Retroviral DNA H structures: displacement-assimilation model of recombination.

The avian retroviruses are unique among known RNA and DNA viruses in their extremely high frequencies of genetic recombination. We propose that these high frequencies can be explained by the facts that the closely associated RNA genomes of this diploid virus can be reverse-transcribed concurrently and that strand displacement is a fundamental property of the reverse transcription reaction. We have elaborated a specific model to describe this process that was suggested by the properties of novel structures observed with high frequency in the electron microscope: DNA duplexes in dimer arrangement that are linked at homologous regions by single-stranded DNA bridges. These structures are presumed to be intermediates of recombination, trapped because their generation in vitro prevents the subsequent resolution steps that would normally take place via the cellular apparatus during infection. The model generates several hypotheses whose exploration should help to test its accuracy.