The F-box cofactor UFO redirects the LEAFY floral regulator to novel cis-elements

Abstract

In angiosperms, flower patterning requires the localized expression of the APETALA3 (AP3) floral homeotic gene involved in petal and stamen development. AP3 is synergistically induced by the master transcription factor (TF) LEAFY (LFY) and the F-box protein UNUSUAL FLORAL ORGANS (UFO), but the molecular mechanism underlying this synergy has remained unknown. Here we show that the connection to ubiquitination pathways suggested by the F-box domain of UFO is mostly dispensable for its function and that UFO instead acts by forming a transcriptional complex with LFY and binds to newly discovered regulatory elements. Cryo-electron microscopy explains how a LFY-UFO complex forms on these novel DNA sites due to direct interaction of UFO with LFY and DNA. Finally, we show that this complex has a deep evolutionary origin, largely predating flowering plants. This work reveals a novel mechanism of an F-box protein in directly modulating the DNA-binding specificity of a master TF.

LEAFY homeostasis is regulated via ubiquitin-dependent degradation and sequestration in cytoplasmic condensates

“… 43 Recent evidence however suggests that the F box domain of UFO is dispensable for UFO-dependent LFY transcriptional activity. 20 UFO indeed belongs to a transcriptional complex that promotes LFY recruitment to novel cis -regulatory elements within the AP3 promoter independently of UFO’s ability to mediate protein degradation. Our work now establishes an extranuclear role for UFO in LFY regulation.…”

“…In summary, our work reveals that LFY undergoes nucleocytoplasmic partitioning to regulate LFY homeostasis via the formation of cytoplasmic biomolecular condensates and UFO-mediated degradation. This partitioning spatially separates the recently described UFO-mediated regulation of LFY transcriptional activity 20 and the UFO-dependent control of cellular LFY levels, allowing cells to faithfully manage both activities without unwanted crosstalk.…”

“…Recently, UFO was shown to form a transcriptional complex with LFY to specify LFY binding to new cis -regulatory elements in the AP3 promoter regions. 20 Interestingly, such a role of UFO appears to be independent of the role of UFO in protein degradation, since it is also observed for the UFOΔF-box variant that does not assemble a functional SCF complex. Therefore, UFO regulates nuclear LFY activity to promote AP3 gene expression.…”

“… 14 , 15 LFY activity on the AP3 promoter is dependent on its interaction with the co-factor UNUSUAL FLOWER ORGANS (UFO). 16 , 17 , 18 , 19 , 20 Ectopic expression of UFO leads to the production of more petals, caused by the ectopic expression of AP3 , but this phenotype is abolished in the absence of LFY. 16 In line with this, ectopic expression of LFY alone is not sufficient to trigger ectopic AP3 expression.…”