酵母菌BRE1同源基因HUB1於阿拉伯芥開花生理調控路徑之功能研究

Abstract

植物配合時令，在適當的時機開花結果是十分重要的。阿拉伯芥在接收各種環境因子之後，再藉由訊息傳導調控FLOWERING LOCUS C (FLC)表現來調控花期。在阿拉伯芥開花訊息傳導的路徑中，FLC扮演著主要的負調控者角色，而它的表現量和FLC啟動子區域的組蛋白修飾有關。本實驗室在阿拉伯芥中分離了一個和酵母菌BER1同源的基因HUB1，它與酵母菌BRE1同樣，具有E3泛素連結酶的功能，且它們的目標都是組蛋白H2B，在酵母菌的研究中發現當H2B接上泛素之後，會招來COMPASS(complex of proteins associated with Set1)複合體，並對附近的另一個組蛋白H3的第四個賴胺酸作三甲基化的修飾(H3K4 tri-methylation)，這樣的組蛋白後修飾組合會促進下游基因的表現。本實驗室發現T-DNA插入突變株hub1具有提早開花的性狀，而控制開花的主要調控者FLOWERING LOCUS C(FLC)的表現量較野生型阿拉伯芥為少。利用染色質免疫沉澱法觀察FLC啟動子區域組蛋白H3K4發生三甲基化之程度，發現若HUB1失去功能，三甲基化程度減少，同時泛素化的程度也降低，我們推測HUB1在阿拉伯芥開花生理調控的角色和酵母菌的BRE1調控基因表現的模式相同，它將單一泛素連結在FLC啟動子區域組蛋白H2B上之後，會促進H3K4發生三甲基化，活化FLC的表現，抑制植物開花，而在hub1植株中，FLC啟動子區域組蛋白活化性修飾減少，FLC表現因此下降，植物失去FLC的抑制，開花即提早發生。

FLOWERING LOCUS C (FLC), a central floral repressor in Arabidopsis thaliana, inhibits the expression of flowering genens, such as FLOWERING LOCUS T (FT) and SUPPRESSOR OF CONSTANT1 (SOC1). One of the mechanisms regulating FLC expression is epigenetic control, which includes histone modifications in gene’s promoter region. We isolated HISTONE MONO-UBIQUITINATION(HUB1) gene from Arabidopsis thaliana, which is a homolog of BRE1 in Saccharomyces cerevisiae. HUB1, with a RING-finger domain in its C-terminus, has E3 ligase activity, while its target, as that of BRE1 in yeast, is histone H2B. In yeast model, ubiquitination on H2B might induce H3K4 tri-methylation via COMPASS complex, and activate gene expression. T-DNA insertion plant with hub1 mutant shows early flowering. Besides, FLC expression level decreases, and its downstream flowering genes, FT and SOC1 are activated in hub1. Furthermore, Chromatin immunoprecipitation (ChIP) was performed, and a dramatic decrease in H3K4 tri-methylation and ubiquitylation was observed in FLC promoter. We propose that HUB1-mediated H2B ubiquitination might induce tri-methylation on H3K4, and consequently activate FLC expression