On the other hand, the Helix 4 tryptophan-to-alanine substitution seriously impaired binding to native DNA in both ARIDs. Much of the mutant protein (about 45–50% in either p270 or Dri) fails to bind to the column and is recovered in the flow-through and wash fractions. The strongly deleterious effect of the tryptophan substitution suggests that the invariable tryptophan plays a critical role in maintaining the overall integrity of the ARID structure in both sequence-specific and sequence-non-specific representatives of the family.
