Blue-light regulation of light-harvesting chlorophyll A/B binding transcript stability

During early plant development environmental signals direct rapid changes in gene expression to establish growth patterns that conform to the immediate surroundings. Changes in gene expression affect morphology physiology and biochemistry to best optimize the plant's ability to grow as an autotroph. One way to regulate gene expression is at the transcript level. RNA degradation regulates accumulation of Light-harvesting chlorophyll a/b binding (Lhcb formerly cab) transcripts. This has been demonstrated in etiolated seedlings, where Lhcb transcript levels increase in response to a short, single low fluence pulse of blue light (104 mumol m-2), but decrease following a pulse of blue-high-fluence light (105 mumol m -2). The decrease in steady-state accumulation is due to transcript destabilization. The 65 base 5'-UTR is necessary and sufficient to confer BHF-mediated destabilization and this response requires the phototropin1 (phot1) photoreceptor and the NPH3 scaffolding protein. In this study, the blue-light mediated regulation of Lhcb transcript stability is examined in the model plant Arabidopsis thaliana. A series of experiments were performed progressing from the phot1 photoreceptor to new proteins participating in transcript destabilization and coalescing in showing the effect of this system in transcript maintenance during diurnal cycles. The study implements phot1 LOV domain mutants to identify the role of the individual chromophore binding domains in the BHF-mediated destabilization process. To identify potential regulatory proteins, a yeast-three-hybrid screen was performed using the Lhcb 5'-UTR as an interaction target. Several bona fide interactors were obtained. One of these proteins is a novel F-box protein designated as KFR1 for Kelch domain, F-box RNA-associated protein. Genetic tests using T-DNA insertion mutants and pharmacological tests using a proteasome inhibitor (MG132) indicate that Kfr1 is required for BHF-induced Lhcb transcript destabilization. To study this destabilization effect in an important biological process outside of de-etiolation, the decay kinetics of Lhcb transcripts under diurnal conditions is studied, and shows a discrete effect of the phot1-KFR1 system on transcript stability. This study advances previous understanding of the blue-light mediated regulation of transcript stability significantly and identifies new components that mediate this response. It addresses the relationship between signal input and changes in physiology to better our understanding in the mechanisms that direct light-regulation of transcript stability.