The frequency of revertants of a single deleterious
nucleotide substitution (G1198A) was studied in viruses obtained from cells
transfected with a cDNA clone (1). Ducklings were inoculated with the recovered
viruses and the wild-type at a 104:1 excess of mutants and, at 25
days post inoculation, the ratio of revertants to wild-type was obtained by
molecular clone sequencing (revertants and the wild-type were distinguishable by
a neutral molecular marker at another site).
This ratio was 0.6 and, correcting for the initial excess of mutants,
the revertant frequency was fs =
6.0 ´ 10-5. This fs value
should equal the initial frequency of revertants in the inocula plus the
frequency of new revertants appeared during replication of the mutant before the
latter was outcompeted. It was estimated
that, at day 25, fs
was 4- to 23-fold higher than the initial fs. Hence, the initial fs ranged from 0.3 ´ 10-5 to 1.5 ´ 10-5.
Multiplying by three to get the mutation frequency to any of the three
nucleotides and taking the geometric mean of the interval bounds, c ´ ms/n/c = 2.0 ´ 10-5. Since
the inoculum came directly from transfected cells, we can assume that the
inoculated viruses had undergone a single cell infection cycle, and thus that ms/n/c =
2.0 ´ 10-5. The
methods used to obtain this estimate are indirect and thus the value should be
taken with caution.
1. Pult,