Regulation of antigenic variation by a DOT1B/RNaseH2 complex in Trypanosoma brucei

染色质的翻译后修饰调节着许多重要的生物过程，如复制、DNA修复和细胞周期控制。我们对单细胞真核寄生虫布鲁氏锥虫如何使用基于染色质的机制来调节抗原变异感兴趣，这是其逃避宿主免疫反应的最重要策略之一。抗原变异是基于一种变异表面糖蛋白（VSG）的单等位基因表达，这种变异表面糖蛋白在寄生虫表面形成一层致密的外壳。寄生虫通过周期性地转换表达的VSG来逃避宿主的免疫反应。然而，介导这些转换事件的确切机制仍然是难以捉摸的。我们对Dot1组蛋白甲基转移酶家族成员在这一过程中的组蛋白甲基化功能特别感兴趣。Dot1（端粒沉默干扰物）最初是在酵母中对过度表达或缺失导致端粒基因抑制的基因进行遗传筛选时发现的。我们最近发现，锥虫体内的两个同源物之一DOT1B与核糖核酸酶RNaseH2蛋白复合物相互作用，从而去除DNA/ rna杂交体，即所谓的r环。有趣的是，锥虫体内RNaseH2活性的丧失会导致r环的积累和DNA损伤，从而导致VSG表达的改变。我们想要阐明DOT1B/RNAseH2复合体在锥虫体内的功能。除了维持基因组完整性的一般功能外，我们特别想验证DOT1B/RNAseH2复合体可能参与VSG调节的假设。

Post-translational modifications of chromatin regulate many important biological processes such as replication, DNA repair and cell cycle control. We are interested in how the unicellular, eukaryotic parasite Trypanosoma brucei uses chromatin-based mechanisms to regulate antigenic variation, one of its most important strategies to evade the host immune response. Antigenic variation is based on monoallelic expression of a variant surface glycoprotein (VSG), which forms a dense coat on the parasite’s surface. The parasites escape the host immune response by periodically switching the expressed VSG. The exact mechanisms, however, which mediate these switching events are still elusive.We are specifically interested in the function of histone methylation by a member of the Dot1 family of histone methyltransferases during this process. Dot1 (disruptor of telomeric silencing) was initially discovered in yeast in a genetic screen for genes whose over-expression or deletion caused derepression of telomeric genes. We recently discovered that DOT1B, one of the two homologues in trypanosomes, interacts with the ribonuclease RNaseH2 protein complex, which removes DNA/RNA-hybrids, so called R-loops. Interestingly, loss of RNaseH2 activity in trypanosomes causes accumulation of R-loops and DNA damage, which leads to altered VSG expression. We want to elucidate the function of the DOT1B/RNAseH2 complex in trypanosomes. In addition to general function in genome integrity maintenance, we specifically want to test the hypothesis that the DOT1B/RNAseH2 complex may be involved in VSG regulation.