As a consequence of being diploid viruses, members of the Retroviridae have
a high recombination rate. To measure recombination between two identical
sequences within the same RNA molecule per round of retroviral replication
cycle, a murine leukemia virus based vector (JZ442 + 3' Hyg) has been const
ructed. It carries a drug resistance gene, hyg, and a 290-bp repeat sequenc
e of the 3' hyg gene inserted into the 3' untranslated region of the green
fluorescent protein gene (gfp). Under fluorescence microscopy, Hyg(r) cells
containing the recombinant proviruses were clear, while a green color was
observed in the drug-resistant cells carrying the parental proviruses, The
rate of recombination was determined by the ratio of the number of clear co
lonies to the total number of Hyg(r) colonies (green and clear colonies). T
he rate of recombination was found to be 62% by this method. The intermolec
ular recombination rate between an infectious virus bearing two copies of t
he 290-bp segment and a noninfectious chimeric RNA virus containing only a
single copy of this sequence was also measured.