The frequency of guanidine-resistant mutants appearing from a guanidine-dependent mutants was measured by plating the virus in the presence and absence of the drug (1). Approximately 2.0 ´ 10⁶ cells were inoculated with ca. 200 plaque forming units (pfu), yielding an average titer of 3.2 ´ 10⁹ pfu/mL in a total volume of 4 mL after completion of the cytopathic effect (1, 2). Hence, the burst size is B = (3.2 ´ 10⁹ ´ 4) / (2.0 ´ 10⁶) = 6400 and using Equation 1 we obtain c = log (4 ´ 3.2 ´ 10⁹ / 200) / log 6400 = 2.1. Drake and Holland (3) gave a similar value (c = 2.5). Sequencing showed that the loss of guanidine dependence could be conferred by each of the three possible nucleotide substitutions at position G4804 or A®G substitution at position 4802 (T = T_s = 4). In two experiments, f_s = 1.1 ´ 10^-4 and f_s = 5.4 ´ 10^-4. Pooling all data f_s = 3.2 ´ 10^-4. Considering that mutations were probably neutral, i.e. a » 1 (although this was not demonstrated), m_s/n/c = 3 ´ 3.2 ´ 10^-4 / 2.1 / 4 = 1.1 ´ 10^-4.

1. de la Torre, J. C., E. Wimmer, and J. J. Holland. 1990. Very high frequency of reversion to guanidine resistance in clonal pools of guanidine-dependent type 1 poliovirus. J. Virol. 64:664-671.

2. Drake, J. W. 1993. Rates of spontaneous mutation among RNA viruses. Proc. Natl. Acad. Sci. USA 90:4171-4175.

3. Drake, J. W. and J. J. Holland. 1999. Mutation rates among RNA viruses. Proc. Natl. Acad. Sci. USA 96:13910-13913.