The evolution of natural proteins is thought to have occurred by succe
ssive fixation of individual mutations. In vitro protein evolution see
ks to accelerate this process, RNA hypermutagenesis, cDNA synthesis in
the presence of biased dNTP concentrations, delivers elevated mutant
and mutation frequencies. Here lineages of active enzymes descended fr
om the homotetrameric 78 residue dihydrofolate reductase (DHFR) encode
d by the Escherichia coli R67 plasmid were generated by iterative RNA
hypermutagenesis, resulting in >20% amino acid replacement, The 22 res
idue N-terminus could be deleted yielding a minimum functional entity
refractory to further changes, designating it as a determinant of R67
robustness, Complete substitution of the segment still allowed fixatio
n of mutations, By the facile introduction of multiple mutations, RNA
hypermutagenesis allows the generation of active proteins derived from
extant genes through a mode unexplored by natural selection.