Structural basis for LEAFY floral switch function and similarity with helix-turn-helix proteins

Abstract

The LEAFY (LFY) protein is a key regulator of flower development in angiosperms. Its gradually increased expression governs the sharp floral transition, and LFY subsequently controls the patterning of flower meristems by inducing the expression of floral homeotic genes. Despite a wealth of genetic data, how LFY functions at the molecular level is poorly understood. Here, we report crystal structures for the DNA-binding domain of Arabidopsis thaliana LFY bound to two target promoter elements. LFY adopts a novel seven-helix fold that binds DNA as a cooperative dimer, forming base-specific contacts in both the major and minor grooves. Cooperativity is mediated by two basic residues and plausibly accounts for LFY's effectiveness in triggering sharp developmental transitions. Our structure reveals an unexpected similarity between LFY and helix-turn-helix proteins, including homeodomain proteins known to regulate morphogenesis in higher eukaryotes. The appearance of flowering plants has been linked to the molecular evolution of LFY. Our study provides a unique framework to elucidate the molecular mechanisms underlying floral development and the evolutionary history of flowering plants.

Prediction of Regulatory Interactions from Genome Sequences Using a Biophysical Model for theArabidopsisLEAFY Transcription Factor

“…Alignment of the 494 unique sequences obtained revealed a 19-bp motif ( Figure 1C), in good agreement with the three-dimensional structure of LFY DBD complexed with DNA (Hamè s et al, 2008). This motif displays the previously established 7-bp consensus as the core.…”

“…From the small number of known LFY DNA binding sites, only a poorly defined 7-bp consensus sequence, CCANTG[G/T], has been previously deduced (Busch et al, 1999;Lamb et al, 2002). The three-dimensional structure of the LFY DNA binding domain has revealed contacts over 19 bp, suggesting considerably greater specificity (Hamè s et al, 2008). Our aim was to capture this specificity in a predictive tool capable of detecting LFY binding sites from plant genomic sequences and ultimately tackle evolutionary questions.…”

LATE MERISTEM IDENTITY2 acts together with LEAFY to activate APETALA1

“…5A). Full length LFY protein homodimerized, as previously proposed (Hames et al, 2008), serving as a positive control. A negative control protein (see Materials and methods for details) did not interact with GST-LFY, confirming the specificity of the observed interactions (Fig.…”

Pushing the frontiers of development

“…Jessica Cande (UC Berkeley, Berkeley, CA, USA) reported two changes in the GRN that underlies heart formation in beetles: one is a gain in an expression domain of tinman; the other is a switch in the expression patterns of two neighboring genes that appears to be morphologically silent. Francois Parcy (CNRS Grenoble, France) discussed insights into the origins of angiospermy that were gained from the analysis of LEAFY -a key regulator of floral development whose DNA-bound structure has recently been determined (Hamès et al, 2008). Homology modeling and biochemical analyses suggest that variations in the DNA-binding specificities of LEAFY and its interactions with other binding partners may have contributed to the appearance of flowers.…”