Deletion of more than 400 amino acids from the carboxyl terminus of an enzyme causes a severe reduction in catalytic activity. Selected point mutations within the residual protein partially reverse the effects of the missing segment. The selection can yield mutants with activities at least ten times as high as those of the starting polypeptides. One well-characterized mutation, a single amino acid replacement in the residual polypeptide, increases the catalytic activity of the polypeptide by a factor of 5. The results suggest substantial potential for design of protein elements to compensate for missing polypeptide sequences. They also may reflect that progenitors of large aminoacyl-tRNA (transfer RNA) synthetases--one of which was used in these studies--were themselves much smaller.