A retroviral vector containing the Herpes
simplex virus tk gene (a neutral mutational target) and
the neo gene was used to score
mutations appearing during a single cell infection cycle by selecting tk null mutants with
bromouridine and total virus-carrying cells with G418 (2), giving f / c = 0.088. According to Drake et al. (1), of 244 tk- mutants, 114 were gross rearrangements and
arose in a mutational target of 2620 bases.
Assuming that all gross rearrangements inactivated the tk gene, the mutation rate to
these changes 0.088 ´ 114 / 244 / 2620 = 1.6 ´ 10-5 rearrangements/s/c. The remaining 130 changes were small
mutations and arose in a target of 1128 bases.
Among 49 small mutants sequenced, 28 were indels. Hence, mi/n/c = 0.088 ´ 130 / 244 ´ 28 / 49 / 1128 = 2.4 ´ 10-5. Among
nucleotide substitutions, three were nonsense mutations. Given that Ts = 76 for nonsense substitutions in this gene, ms/n/c
= 3 ´ 0.088 ´ 130 / 244 ´ 3 / 49 / 76 = 1.1 ´ 10-4. The
indel fraction is thus d = (1.6 ´
10-5 + 2.4 ´
10-5) / (1.1 ´
10-4 + 1.6 ´
10-5 + 2.4 ´
10-5) = 0.27.
1. Drake, J. W., B. Charlesworth,
D. Charlesworth, and J. F. Crow. 1998. Rates of
spontaneous mutation. Genetics 148:1667-1686.
2. Parthasarathi, S., A. Varela-Echavarria,
Y. Ron, B. D. Preston, and J. P. Dougherty. 1995. Genetic rearrangements
occurring during a single cycle of murine leukemia
virus vector replication: characterization and implications. J. Virol. 69:7991-8000.