Table 1.

Effect of amino acid substitutions in the exonuclease domain on spontaneous mutation rates and exonuclease activity

Enzyme	Mutator activity*	Exonuclease activity† (ssDNA)
Wild type	1	1
G255S	58	0.9
D131N	15	0.7
D131G	250	0.2
D112A + E114A	300	4  ×  10−5

^*Mutator activity was measured by determining the reversion frequency of an rII⁻ mutant to wild type or pseudo-wild type (14, 18). The mutator activity values are relative to the wild-type reversion frequency. 

^†Exonuclease activity was measured with single-stranded ³H-labeled DNA as described (14, 18) and is expressed relative to the activity measured for the wild-type enzyme.