Transcriptional regulation is a complex and pivotal process in living cells. HOS15 is a transcriptional corepressor. Although transcriptional repressors generally have been associated with inactive genes, increasing evidence indicates that, through poorly understood mechanisms, transcriptional corepressors also associate with actively transcribed genes. Here, we show that HOS15 is the substrate receptor for an SCF/CUL1 E3 ubiquitin ligase complex (SCF^HOS15) that negatively regulates plant immunity by destabilizing transcriptional activation complexes containing NPR1 and associated transcriptional activators. In unchallenged conditions, HOS15 continuously eliminates NPR1 to prevent inappropriate defense gene expression. Upon defense activation, HOS15 preferentially associates with phosphorylated NPR1 to stimulate rapid degradation of transcriptionally active NPR1 and thus limit the extent of defense gene expression. Our findings indicate that HOS15-mediated ubiquitination and elimination of NPR1 produce effects contrary to those of CUL3-containing ubiquitin ligase that coactivate defense gene expression. Thus, HOS15 plays a key role in the dynamic regulation of pre- and postactivation host defense.

转录调控是活细胞中一个复杂而关键的过程。HOS15是转录共表达因子。尽管转录阻遏物通常与失活的基因相关联，但越来越多的证据表明，通过不充分了解的机制，转录共抑制物也与活跃转录的基因相关联。在这里，我们显示HOS15是SCF / CUL1 E3泛素连接酶复合物（SCF ^HOS15）通过破坏包含NPR1和相关转录激活因子的转录激活复合物来稳定调节植物的免疫力。在无挑战的条件下，HOS15持续消除NPR1，以防止防御基因表达不当。在防御激活后，HOS15优先与磷酸化的NPR1结合以刺激转录活性NPR1的快速降解，从而限制了防御基因表达的程度。我们的发现表明，HOS15介导的泛素化和NPR1的消除产生的作用与共同激活防御基因表达的含CUL3的泛素连接酶相反。因此，HOS15在激活前和激活后宿主防御的动态调节中起着关键作用。