Understanding genomic-context specific deposition and function of H3.V and H4.V histone variants in Trypanosoma brucei

在许多生物体中的研究已经表明，用组蛋白变体替换典型组蛋白可以影响DNA可及性。因此，组蛋白变体沉积代表了调节任何DNA模板过程（例如转录、复制或DNA修复）的手段。大量不同的组蛋白变体和复杂的全基因组染色质模式已经减缓了对导致组蛋白变体在特定基因组位点沉积的机制以及基因组背景在确定组蛋白变体功能中的作用的全面理解的进展。因此，对于大多数组蛋白变异体来说，人们还不清楚它们是如何以及为什么被靶向到特定的基因组位点的，以及它们是否可以根据它们的基因组背景而具有不同的功能。布氏杆菌是一种单细胞真核生物，具有许多特征，使其非常适合研究组蛋白变异生物学的基本机制。T.布氏杆菌的每个典型组蛋白只有一个变体，并且由于基因排列成多顺反子转录单位（PTU），只有相对较少的数量，仅约200个转录起始位点（TSS）和转录终止位点（TTS）。此外，我们还证明了在T.布氏杆菌中H2A.Z、H2.V、H3.V和H4.V四种不同的组蛋白变体在全基因组中的分布非常不同，这使得很容易检测到变体分布中的微小变化。此外，我们已经观察到，同时删除H3.V和H4.V强烈影响染色质压实在特定的基因组loci和增加抗原转换的recombination.Based基于这些发现，我们假设，根据其基因组的情况下，H3.V有不同的生物学功能。我们怀疑它沉积在双链断裂的位点以帮助DNA修复，并沉积在PTU的末端以确保正确的转录终止。为了验证这些假设，我们将研究H3.V如何以及为什么靶向特定的基因组位点，并确定基因组背景是否影响它在局部染色质结构和高阶基因组组织中的作用。

Studies in numerous organisms have shown that the replacement of canonical histones with histone variants can affect DNA accessibility. Thus, histone variant deposition represents a means to regulate any DNA-templates process, such as transcription, replication or DNA repair. A large number of different histone variants and complex genome-wide chromatin patterns have slowed progress towards a comprehensive understanding of the mechanisms leading to the deposition of histone variants at specific genomic loci and the role of the genomic context in determining the function of histone variants. Thus, for most histone variants it is not understood how and why they are targeted to specific genomic locus and whether they can have different functions depending on their genomic context.To overcome these hurdles, we propose to study histone variant deposition and function in T. brucei, a unicellular eukaryote with many features that make it ideally-suited to investigate basic mechanisms of histone variant biology. T. brucei possess only one variant for each canonical histone and, due to the arrangement of genes into polycistronic transcription units (PTUs), a relatively small number of only ~200 transcription start sites (TSSs) and transcription termination sites (TTSs). In addition, we have shown that in T. brucei the four different histone variants, H2A.Z, H2.V, H3.V and H4.V, exhibit a very distinct genome-wide distribution, making it easy to detect small changes in variant distribution.Recently, we have found that loss of H3.V leads to a defect in transcription termination and changes in genome organization. In addition, we have observed that concurrent deletion of H3.V and H4.V strongly affects chromatin compaction at specific genomic loci and increases antigen switching by recombination.Based on these findings we hypothesize that, depending on its genomic context, H3.V has different biological functions. We suspect that it is deposited at sites of double stranded break to aid in DNA repair and at the end of PTUs to ensure proper transcription termination. To test these hypotheses, we will investigate how and why H3.V is targeted to specific genomic loci and determine whether the genomic context affects the role it plays in local chromatin structure and higher order genome organization.