Single-subunit polymerases are universally encoded in both cellular organisms and viruses. Their three-dimensional structures have the shape of a right-hand with the active site located in the palm region, which has a topology similar to that of the RNA recognition motif (RRM) found in many RNA-binding proteins. Considering that polymerases have well conserved structures, it was surprising that the RNA-dependent RNA polymerases from birnaviruses, a group of dsRNA viruses, have their catalytic motifs arranged in a permuted order in sequence. Here we report the 2.5 Å structure of a birnavirus VP1 in which the polymerase palm subdomain adopts a new active site topology that has not been previously observed in other polymerases. In addition, the polymerase motif C of VP1 has the sequence of -ADN-, a highly unusual feature for RNA-dependent polymerases. Through site-directed mutagenesis, we have shown that changing the VP1 motif C from -ADN-to -GDD-results in a mutant with an increased RNA synthesis activity. Our results indicate that the active site topology of VP1 may represent a newly developed branch in polymerase evolution, and that birnaviruses may have acquired the -ADN-mutation to control their growth rate.evolution ͉ virus ͉ RdRp S ingle-subunit polymerases, including RNA/DNA-dependent RNA/DNA polymerases, are universally encoded in cellular organisms and viruses. Sequence analysis shows that RNAdependent RNA polymerases (RdRps) are a cluster of closely related enzymes that are mostly found in viruses, in which they assume critical functional roles by replicating and transcribing viral genome. The recently reported crystal structures of several RdRps and their functional complexes have provided important insights into the biological functions and catalytic mechanisms of these enzymes (1-10). These structures contain a well conserved core polymerase domain, the shape of which resembles a right hand with fingers, palm, and thumb subdomains. The palm, which is the most conserved region, contains a central, nonvariant, four-stranded -sheet with five recurring catalytic sequence motifs (from A to E).In contrast to these conventional features, birnavirus polymerase VP1 exhibits several unusual characteristics (11,12). Most noticeably, the essential -X(G)DD-sequence, which is often referred to as the polymerase motif C (13, 14), is missing from VP1 (12). This raises the question as to whether VP1 promotes catalysis by the two-metal mechanism like in conventional polymerases, or whether VP1 employs a different mechanism for nucleotidyl transfer. Recent results indicate that birnavirus VP1, as well as the polymerases from some tetraviruses, may belong to a special group of unconventional polymerases with five essential RNA polymerase motifs arranged in the permuted order of C-A-B-D-E (15). In addition, the motif C in birnaviruses may have the sequence -ADN-, resulting in only two aspartate residues in the active site (15).Like polymerases from other dsRNA viruses, birnavirus VP1 catalyzes both replication and transc...