Structural Evolution of LEAFY Reveals DNA-Mediated Cooperativity and Dimerization Shifts at the Water-to-Land Transition.

The evolution of transcription factor (TF) DNA-binding specificity is a major driver of gene regulatory innovation. Unlike most TFs, which diversify through gene duplication and neofunctionalization, the plant-specific LEAFY (LFY) TF evolved novel binding specificities without extensive duplication. Here, we combine experimental structural determination and biochemical assays to reveal how LFY's dimerization and DNA-binding preferences shifted during the water-to-land transition. We present crystal structures of the LFY DNA-binding domain (DBD) from the hornwort Nothoceros aenigmaticus and the alga Interfilum paradoxum bound to DNA, demonstrating two distinct dimerization mechanisms: one mediated by direct protein-protein interactions and another driven by DNA-mediated cooperativity. In the ancestral state, LFY likely bound DNA as a dimer through DNA-mediated cooperativity, with protein-protein dimerization emerging later, enforcing new DNA-binding preferences. Our findings support a revised evolutionary scenario for LFY, highlighting the dynamic interplay between protein-DNA and protein-protein interactions as key drivers of TF binding specificity. This work deepens our understanding of how structural adaptations in TFs underpin evolutionary transitions in gene regulation.