Revisiting chromatin binding of the Arabidopsis UV-B photoreceptor UVR8.

Melanie Binkert,Richard Chappuis,Sarah Raffelberg,Kelvin Lau,Carlos D Crocco,Babatunde Ekundayo,Thomas Schalch,Ruohe Yin,Kimberley Tilbrook,Roman Ulm

doi:10.1186/s12870-016-0732-5

Abstract

BackgroundPlants perceive UV-B through the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor and UVR8 activation leads to changes in gene expression such as those associated with UV-B acclimation and stress tolerance. Albeit functionally unrelated, UVR8 shows some homology with RCC1 (Regulator of Chromatin Condensation 1) proteins from non-plant organisms at the sequence level. These proteins act as guanine nucleotide exchange factors for Ran GTPases and bind chromatin via histones. Subsequent to the revelation of this sequence homology, evidence was presented showing that UVR8 activity involves interaction with chromatin at the loci of some target genes through histone binding. This suggested a UVR8 mode-of-action intimately and directly linked with gene transcription. However, several aspects of UVR8 chromatin association remained undefined, namely the impact of UV-B on the process and how UVR8 chromatin association related to the transcription factor ELONGATED HYPOCOTYL 5 (HY5), which is important for UV-B signalling and has overlapping chromatin targets. Therefore, we have investigated UVR8 chromatin association in further detail.ResultsUnlike the claims of previous studies, our chromatin immunoprecipitation (ChIP) experiments do not confirm UVR8 chromatin association. In contrast to human RCC1, recombinant UVR8 also does not bind nucleosomes in vitro. Moreover, fusion of a VP16 activation domain to UVR8 did not alter expression of proposed UVR8 target genes in transient gene expression assays. Finally, comparison of the Drosophila DmRCC1 and the Arabidopsis UVR8 crystal structures revealed that critical histone- and DNA-interaction residues apparent in DmRCC1 are not conserved in UVR8.ConclusionThis has led us to conclude that the cellular activity of UVR8 likely does not involve its specific binding to chromatin at target genes.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0732-5) contains supplementary material, which is available to authorized users.

Highlights

Plants perceive UV-B through the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor and UVR8 activation leads to changes in gene expression such as those associated with UV-B acclimation and stress tolerance
In contrast to previous reports [6, 18,19,20], we did not detect UVR8 chromatin association with HYPOCOTYL 5 (HY5) using the antiUVR8(410–424) antibody in conjuction with several different probes covering a large portion of the HY5 genomic region (Fig. 1c and Additional file 1a), including those regions previously reported to be bound by UVR8
It has been rather well established that UVR8 activity requires UV-B-dependent interaction with CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), which leads to HY5 stabilization [7,8,9, 20]

Summary

Introduction

Plants perceive UV-B through the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor and UVR8 activation leads to changes in gene expression such as those associated with UV-B acclimation and stress tolerance. UVR8 shows some homology with RCC1 (Regulator of Chromatin Condensation 1) proteins from non-plant organisms at the sequence level. These proteins act as guanine nucleotide exchange factors for Ran GTPases and bind chromatin via histones. Subsequent to the revelation of this sequence homology, evidence was presented showing that UVR8 activity involves interaction with chromatin at the loci of some target genes through histone binding. This suggested a UVR8 mode-of-action intimately and directly linked with gene transcription. RCC1 proteins act as guanine nucleotide exchange factors (GEF) and regulate the small GTPase

Methods

Results

Conclusion