An A® G mutant at position 40 from the 3´end of the genome was obtained by site-directed mutagenesis, plaque-purified and propagated a high multiplicity of infection (moi), such that each passage corresponded to a single infection cycle (2). The fraction of revertants to the wild-type was measured after each passage, up to 10 passages, by T1 RNAase fingerprinting. A system of linear equations was used to estimate the fraction of revertant phage produced per passage, accounting for the selective disadvantage of the mutant. This gave a most likely value of f_s / c = 3.5 ´ 10^-4 revertants per passage (1). Since T_s = 1, Equation 2 gives m_s/n/c = 3 ´ 3.5 ´ 10^-4 = 1.1 ´ 10^-3. Transitions are more likely than transversions. Therefore, this might be an overestimation. Also, a mutation rate of 1.1 ´ 10^-3 s/n/c corresponds to more than four mutations per genome, which would probably impose an excessive mutational load to the virus. For this reason, this estimate was used initially by Drake (3) but discarded later (4, 5).

1. Batschelet, E., E. Domingo, and C. Weissmann. 1976. The proportion of revertant and mutant phage in a growing population, as a function of mutation and growth rate. Gene 1:27-32.

2. Domingo, E., R. A. Flavell, and C. Weissmann. 1976. In vitro site-directed mutagenesis: generation and properties of an infectious extracistronic mutant of bacteriophage Qbeta. Gene. 1:3-25.

3. Drake, J. W. 1993. Rates of spontaneous mutation among RNA viruses. Proc. Natl. Acad. Sci. U. S. A. 90:4171-4175.

4. Drake, J. W., B. Charlesworth, D. Charlesworth, and J. F. Crow. 1998. Rates of spontaneous mutation. Genetics 148:1667-1686.

5. Drake, J. W. and J. J. Holland. 1999. Mutation rates among RNA viruses. Proc. Natl. Acad. Sci. U. S. A 96:13910-13913.