The recombination rate is not increased when retroviral RNA is missing an encapsidation sequence.

Retroviruses, as a result of the presence of two identical RNA molecules in their virions, recombine at a high rate. When nonhomologous RNA is present in the dimer RNA molecules, nonhomologous recombination occurs, although the rate is only 0.1% of the rate of homologous recombination. We developed a protocol to study transduction of cellular proto-oncogenes in a single cycle of retrovirus replication. The psi sequences is a cis required sequence for packaging viral RNA into viral particles. To test if the rate of nonhomologous recombination increases about 1,000 times when the psi sequence is deleted, as reported by other, we modified vectors we used previously (J. Zhang and H. M. Temin, Science 259:234-238, 1993). Our results indicated that the recombination rate did not undergo the increase of about 1,000 times when the psi sequence of a chimeric RNA was deleted.