Casein kinase I (CK1), a ubiquitous Ser/Thr protein kinase in eukaryotes, plays a critical role in higher plant flowering. Arabidopsis CK1 family member MUT9‐LIKE KINASEs, such as MLK1 and MLK3, have been shown to phosphorylate histone H3 at threonine 3 (H3T3), an evolutionarily conserved residue, and the modification is associated with the transcriptional repression of euchromatic and heterochromatic loci. This study demonstrates that mlk4‐3, a T‐DNA insertion mutant of MLK4, flowered late, and that overexpression of MLK4 caused early flowering. The nuclear protein MLK4 phosphorylated histone H3T3 both in vitro and in vivo, and this catalytic activity required the conserved lysine residue K175. mutation of MLK4 at K175 failed to restore the level of phosphorylated H3T3 (H3T3ph) or to complement the phenotypic defects of mlk4‐3. The FLC/MAF‐clade genes, including FLC, MAF4 and MAF5, were significantly upregulated in mlk4‐3. The double mutant mlk4‐3 flc‐3 flowered earlier than mlk4‐3, suggesting that functional FLC is crucial for flowering repression in mlk4‐3. Chromatin immunoprecipitation assays showed that MLK4 bound to FLC/MAF chromatin and that H3T3ph occupancy at the promoter of FLC/MAF was negatively associated with its transcriptional level. In accordance, H3T3ph accumulated at FLC/MAF in 35S::MLK4/mlk4‐3 but diminished in 35S::MLK4(K175R)/mlk4‐3 plants. Moreover, the amount of RNA Pol II deposited at FLC/MAF was clearly enriched in mlk4‐3 relative to the wild type. Therefore, MLK4‐dependent phosphorylation of H3T3 contributes to accelerating flowering by repressing the transcription of negative flowering regulator FLC/MAF. This study sheds light on the delicate control of flowering by the plant‐specific CK1, MLK4, via post‐translational modification of histone H3.

中文翻译：

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MLK4介导的组蛋白H3T3磷酸化通过拟南芥FLC / MAF的转录沉默促进开花

酪蛋白激酶I（CK1）是真核生物中普遍存在的Ser / Thr蛋白激酶，在植物高花期中起关键作用。拟南芥CK1家族成员MUT9-like激酶（如MLK1和MLK3）已在苏氨酸3（H3T3）（一种进化上保守的残基）上磷酸化组蛋白H3，该修饰与常染色体和异染色体位点的转录抑制有关。这项研究证明了MLK4的T-DNA插入突变体mlk4-3开花较晚，而MLK4的过表达导致早期开花。核蛋白MLK4磷酸化组蛋白H3T3都在体外和体内，并且此催化活性需要保守的赖氨酸残基K175。MLK4在K175的突变未能恢复磷酸化H3T3（H3T3ph）的水平或补充mlk4-3的表型缺陷。的FLC / MAF -clade基因，包括FLC，MAF4和MAF5，在被显著上调mlk4-3。双突变体mlk4-3 flc-3的开花早于mlk4-3，这表明功能性FLC对于mlk4-3中的开花抑制至关重要。染色质免疫沉淀试验表明MLK4与FLC / MAF结合染色质和在FLC / MAF启动子处的H3T3ph占用与其转录水平呈负相关。因此，H3T3ph在35S :: MLK4 / mlk4-3中在FLC / MAF处积累，但在35S :: MLK4（K175R）/ mlk4-3植物中有所减少。此外，相对于野生型，沉积在FLC / MAF上的RNA Pol II的量显然富含mlk4-3。因此，依赖MLK4的H3T3磷酸化通过抑制阴性开花调节因子FLC / MAF的转录来促进开花。这项研究揭示了通过组蛋白H3的翻译后修饰，植物特异性CK1，MLK4对开花的精细控制。