A plaque-purified thermosensitive mutant
(C5310U) was plated at 33°C and 39°C to obtain the frequency of
revertants to the wild-type (1). This
mutation was approximately neutral at 33°C (a » 1) and only the C to U reversion
restored growth at 39°C (T = Ts =
1). The average revertant frequency in
three isolated plaques was fs =
3.1 ´ 10-5 and c = 2.8 (2). Hence ms/n/c = 3 ´ 3.1 ´ 10-5 / 2.8 = 3.3 ´ 10-5. In a
second experiment, the isolated plaques were passaged once in liquid culture
and the observed revertant frequency was fs = 2.3 ´ 10-5. In the
first experiment, the total number of viruses was 1.1 ´ 109 and therefore, using
Equation 1 with N0 = 1, we
obtain B = (1.1 ´ 109)1/2.8 = 1694. In the second experiment, a 10-5 dilution was applied to inoculate the liquid
culture and the average number of viruses after growth was 8.4 ´ 109. Hence the amplification factor was (8.4 ´ 109) / (1.1 ´ 109) ´ 105 = 7.6 ´ 105, which corresponds to log (7.6 ´ 105) / log 1694 = 1.8 additional
infection cycles. Hence, ms/n/c =
3 ´ 2.3 ´ 10-5 / (2.8 + 1.8) = 1.5 ´ 10-5. Taking
the geometrical mean of the estimates from the two experiments, ms/n/c =
2.2 ´ 10-5.
1. de la Torre, J.
C., C. Giachetti, B. L. Semler,
and J. J. Holland. 1992. High frequency of single-base transitions and
extreme frequency of precise multiple-base reversion mutations in poliovirus.
Proc. Natl. Acad. Sci. USA 89:2531-2535.
2. Drake, J. W. and J. J.
Holland. 1999. Mutation rates among RNA viruses. Proc. Natl.
Acad. Sci. USA
96:13910-13913.