Function of NOT9B and the CCR4-NOT complex in phytochrome A dependent light signalling

光敏色素（Phytochromes）是植物中的红色/远红色光感受器，在植物生长发育适应环境变化中起着关键作用。在大多数植物中，光敏色素由小的基因家族代表。PhyA和phyB是种子植物中主要的光敏色素，参与调节种子萌发、去黄化、诱导开花和对遮荫的响应等生长发育过程。PhyB是红光的主要受体，而phyA是感知远红光和非常低强度光所必需的。当光激活时，光敏色素从细胞质转运到细胞核并调节基因表达。光敏色素下游有两种抑制光反应的信号通路。光敏色素相互作用因子（pif）是bHLH转录因子的一个亚组，负性调节光信号。与此同时，PHYA-105 (COP1/SPA) E3泛素连接酶复合物的组成型光形态形成1/抑制因子靶向光信号的正调节因子，使其被蛋白酶体降解。光激活光敏色素抑制pif和COP1/SPA，从而诱导对光的反应。为了确定光敏色素下游可能独立于pif和COP1/SPA的新信号通路，我们对与光激活phyA相互作用的蛋白进行了酵母双杂交筛选。在这个屏幕中，我们发现了NOT9B，它是CCR4-NOT复合物的一个组成部分。CCR4-NOT复合物在酵母、果蝇和哺乳动物中具有相当好的特征，并在RNA代谢和基因表达调控的不同方面发挥作用。然而，人们对CCR4-NOT在植物中的功能知之甚少。我们发现NOT9B是植物介导的光反应的负调节因子，我们提出NOT9B与CCR4-NOT相关，抑制光信号。初步数据显示，光激活的phyA结合NOT9B，从而取代CCR4-NOT中的NOT9B，以减轻phyA依赖性光信号的抑制。在本项目中，我们希望进一步研究phyA调控CCR4-NOT的分子机制，并了解CCR4-NOT如何调节光控基因的表达。

Phytochromes (phys) are red/far-red photoreceptors in plants that play a critical role in adaptation of growth and development to changes in the environment. In most plants, phytochromes are represented by small gene families. PhyA and phyB are the main phytochromes in seed plants involved in regulation of growth and development, including seed germination, de-etiolation, induction of flowering, and responses to canopy shade. PhyB is the primary receptor for red light, while phyA is required for sensing far-red light and light of very low intensity.When activated by light, phytochromes translocate from the cytosol into the nucleus and regulate gene expression. There are two well-investigated signalling pathways downstream of phytochromes that inhibit light responses. The PHYTOCHROME INTERACTING FACTORs (PIFs), a subgroup of the bHLH transcription factors, negatively regulate light signalling. In parallel, the CONSTITUTIVELY PHOTOMORPHOGENIC 1/SUPPRESSOR OF PHYA-105 (COP1/SPA) E3 ubiquitin ligase complex targets positive regulators of light signalling for degradation by the proteasome. Light-activated phytochromes inhibit both PIFs and COP1/SPA and thereby induce responses to light.To identify novel signalling pathways downstream of phytochromes that are potentially independent of PIFs and COP1/SPA, we performed a yeast two hybrid screen for proteins that interact with light-activated phyA. In this screen we found NOT9B, a component of the CCR4-NOT complex. The CCR4-NOT complex is comparably well characterised in yeast, Drosophila, and mammals and plays a role in different aspects of RNA metabolism and regulation of gene expression. However, much less is known about the function of CCR4-NOT in plants. We show that NOT9B is a negative regulator of phyA-mediated light responses and we propose that NOT9B associated with CCR4-NOT represses light signalling. Preliminary data show that light-activated phyA binds NOT9B and thereby displaces NOT9B from CCR4-NOT to relieve repression of phyA-dependent light signalling. In the proposed project we want to further investigate the molecular mechanisms underlying regulation of CCR4-NOT by phyA and understand how CCR4-NOT modulates expression of light-controlled genes.