CrAss-like phages are a recently described expansive group of viruses that includes the most abundant virus in the human gut 1 – 3 . The genomes of all crAss-like phages encode a large virion-packaged protein 2 , 4 that contains a DFDxD sequence motif, which forms the catalytic site in cellular multisubunit RNA polymerases (RNAPs) 5 . Here, using Cellulophaga baltica crAss-like phage phi14:2 as a model system, we show that this protein is a DNA-dependent RNAP that is translocated into the host cell along with the phage DNA and transcribes early phage genes. We determined the crystal structure of this 2,180-residue enzyme in a self-inhibited state, which probably occurs before virion packaging. This conformation is attained with the help of a cleft-blocking domain that interacts with the active site and occupies the cavity in which the RNA–DNA hybrid binds. Structurally, phi14:2 RNAP is most similar to eukaryotic RNAPs that are involved in RNA interference 6 , 7 , although most of the phi14:2 RNAP structure (nearly 1,600 residues) maps to a new region of the protein fold space. Considering this structural similarity, we propose that eukaryal RNA interference polymerases have their origins in phage, which parallels the emergence of the mitochondrial transcription apparatus 8 . The RNA polymerase from the crAss-like bacteriophage phi14:2, which is translocated into the host cell with phage DNA and transcribes early phage genes, is structurally most similar to eukaryotic RNA interference polymerases, suggesting that the latter have a phage origin.

中文翻译：

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crAss样噬菌体病毒粒子RNA聚合酶的结构和功能

CrAss 样噬菌体是最近描述的一大类病毒，其中包括人类肠道中最丰富的病毒 1 – 3 。所有 crAss 样噬菌体的基因组都编码一种大型病毒粒子包装蛋白 2 , 4 ，其中包含 DFDxD 序列基序，该基序形成细胞多亚基 RNA 聚合酶 (RNAP) 5 中的催化位点。在这里，使用 Cellulophaga baltica crAss 样噬菌体 phi14:2 作为模型系统，我们表明该蛋白质是一种 DNA 依赖性 RNAP，与噬菌体 DNA 一起易位到宿主细胞中并转录早期噬菌体基因。我们确定了这种具有 2,180 个残基的酶在自我抑制状态下的晶体结构，这可能发生在病毒粒子包装之前。这种构象是在裂口阻断结构域的帮助下获得的，该结构域与活性位点相互作用并占据 RNA-DNA 杂交体结合的空腔。在结构上，phi14:2 RNAP 与参与 RNA 干扰的真核 RNAP 最相似 6 , 7 ，尽管大多数 phi14:2 RNAP 结构（近 1,600 个残基）映射到蛋白质折叠空间的新区域。考虑到这种结构相似性，我们提出真核RNA干扰聚合酶起源于噬菌体，这与线粒体转录装置的出现平行 8 。来自crAss样噬菌体phi14:2的RNA聚合酶与噬菌体DNA一起易位到宿主细胞中并转录早期噬菌体基因，其结构与真核RNA干扰聚合酶最相似，表明后者具有噬菌体起源。