The reversion frequency of an amber mutant was
measured by plating the virus in permissive and selective cells (1). Only substitution UAG to UCG was viable (T = Ts
= 1), and selection against the amber mutant was strong. Despite selection, it is still possible to
use the null-class method to estimate the mutation rate. Out of 27 independent cultures assayed, 23
showed no revertants (P0 =
23/27). The average number of pfu per culture was 1.8 ´ 104 but the plating
efficiency of the amber mutant was approximately three times lower than that of
the revertant, so the corrected number is 5.4 ´ 104 pfu. Hence, using Equation 10, ms/n/r =
- 3 ´ log (23 / 27) / (5.4 ´ 104) = 8.9 ´ 10-6.
1. Sedivy, J. M., J. P. Capone, U. L. RajBhandary, and P. A. Sharp. 1987. An inducible
mammalian amber suppressor: propagation of a poliovirus mutant. Cell 50:379-389.