Operator-constitutive mutations in a DNA sequence recognized by a yeast homeodomain.

Homeodomain proteins regulate transcription in organisms as diverse as yeasts, mammals and plants, often effecting key decisions in development. Although homeodomains can selectively recognize certain DNA sequences, a question has arisen as to how specific this interaction is and how much it contributes to the ability of these proteins to properly select target genes in the cell. This question is particularly an issue in cases where the homeodomain proteins recognize DNA cooperatively with other DNA-binding proteins. In this paper, we examine the issue of DNA binding specificity for the homeodomain of the yeast alpha 2 protein (which recognizes the a-specific gene operator cooperatively with the MCM1 protein) by examining both in vivo and in vitro the effects of point mutations in its recognition sequence. We found that most changes in the homeodomain recognition sequence produced only small effects on both homeodomain affinity as measured in vitro (with and without the helper protein MCM1) and operator function as determined in vivo. This tolerance for operator mutations illustrates in a systematic way the modest DNA-binding specificity of the alpha 2 homeodomain and contrasts with the behavior of many of the bacterial and phage repressors where single point mutations in the operator can have dramatic effects on affinity. This tolerance for different sequences may arise from the fact that most of the interactions made between the alpha 2 homeodomain and the DNA occur through long amino acid side chains; we suggest that these side chains can reconfigure in order to create surfaces complementary to many different DNA sequences. The relaxed DNA-binding specificity of homeodomain proteins such as alpha 2 may be an important feature that permits new regulatory circuits to evolve rapidly from existing components.