p270 and Dri showed a different tolerance to the third mutation, a tyrosine-to-alanine substitution in Helix-5. The elution profile of the p270 mutant peptide is similar to that of the wild-type peptide. Approximately the same amount of signal is retained on the column, although the shift in the elution peak from the second to the first 200 mM fraction indicates a weakening of affinity. This type of elution profile suggests that the substitution causes loss of one or more DNA contact sites, but does not suggest that protein folding is grossly affected. In contrast, the corresponding substitution in Dri is as deleterious as the tryptophan substitution, with 40–50% of the signal failing to bind to the column, implying that this residue is critical in the Dri ARID for maintaining proper structure.
