Genetic recombination is a major force driving the evolution of many viruses.
Recombination between two copackaged retroviral genomes may occur at rates
as high as 40% per replication cycle1. This enables genetic
information to be shuffled rapidly, leading to recombinants with new patterns
of mutations and phenotypes. The in vitro process of DNA shuffling2,
3 (molecular breeding) mimics this mechanism on a vastly parallel
and accelerated scale. Multiple homologous parental sequences are recombined
in parallel, leading to a diverse library of complex recombinants from which
desired improvements can be selected. Different proteins and enzymes have
been improved using DNA shuffling4,
5,
6. We report here the
first application of molecular breeding to viruses. A single round of shuffling
envelope sequences from six murine leukaemia viruses (MLV) followed by selection
yielded a chimaeric clone with a completely new tropism for Chinese Hamster
Ovary (CHOK1) cells. The composition and properties of the selected clone
indicated that this particular permutation of parental sequences cannot be
readily attained by natural retroviral recombination. This example demonstrates
that molecular breeding can enhance the inherently high evolutionary potential
of retroviruses to obtain desired phenotypes. It can be an effective tool,
when information is limited, to optimize viruses for gene therapy and vaccine
applications when multiple complex functions must be simultaneously balanced.
