Setting a circadian clock: Structural understanding of the ligand-activation model of the circadian evening complex components ELF4 to ELF3

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The plant circadian clock drives genome-scale transcription to coordinate most of growth and development. EARLY FLOWERING 3 (ELF3) is the key hub for this diurnal signalling. The Davis group recently showed that ELF3 acts as a receptor integrator for the ELF4 ligand, together termed the Evening Complex, to initiate this foundationally important repressor complex. It was also shown that light represses this action, leading to a hypothesis for the mechanism by which increases in light intensity speed up the clock; light signalling acts as a repressor of a repressor. As ELF3 encodes a protein of unknown activity, it has remained enigmatic how it performs its key repressive role. Here we propose a project to unravel a clock-resetting mechanism by ELF3. Using a combination of biochemical, cellular and structural experiments, one would examine the spatial-temporal function of ELF3. One would monitor the global, genome-wide binding of this chromatin-associated factor to define its transcriptional target genes. Next an exploration of the localisation context of where ELF4 activates ELF3, and how light-perception represses this would be examined. Efforts to solve the co-complex of ELF4 and ELF3 will be undertaken and with associated biophysics of complex assembly, how ELF4 association alters ELF3 folding will be elucidated. As ELF3 is known to be bound and repressed by a photoreceptor, activation of ELF3 by ELF4 will be examined in a testable context that supports "photoreceptor-relief" of this key clock factor. Taken together this project is envisaged to provide one mechanistic basis of a long-standing problem in the clock community termed entrainment, where light perception leads to acceleration of periodicity. This is the first ever description of a clock-resetting mechanism in plants.