Structure and function of virion RNA polymerase of crAss-like phage

CrAss-like phages are a recently described family-level group of viruses that includes the most abundant virus in the human gut[1][1],[2][2]. Genomes of all crAss-like phages encode a large virion-packaged protein[2][2],[3][3] that contains a DFDxD sequence motif, which forms the catalytic site in cellular multisubunit RNA polymerases (RNAPs)[4][4]. Using Cellulophaga baltica crAss-like phage phi14:2 as a model system, we show that this protein is a novel 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, likely pre-virion-packaged state. This conformation is attained with the help of a Cleft-blocking domain that interacts with the active site motif and occupies the RNA-DNA hybrid binding grove. Structurally, phi14:2 RNAP is most similar to eukaryotic RNAPs involved in RNA interference[5][5],[6][6], although most of phi14:2 RNAP structure (nearly 1,600 residues) maps to a new region of protein folding space. Considering the structural similarity, we propose that eukaryal RNA interference polymerases take their origin in a phage, which parallels the emergence of the mitochondrial transcription apparatus[7][7]. [1]: #ref-1 [2]: #ref-2 [3]: #ref-3 [4]: #ref-4 [5]: #ref-5 [6]: #ref-6 [7]: #ref-7