Figure 1.

Bacteriophage φ6 RdRP performs replication and transcription. (A) A schematic of the φ6 RdRP structure. A nucleotidyl transfer active site is linked to the enzyme surface via three tunnels: the template entry tunnel, the NTP tunnel, and the product exit tunnel. (B) During replication, a complementary antisense RNA strand [(−)RNA; red line] is polymerized onto a sense RNA template [(+)RNA; black line]. The 3′-end of the template strand accesses the enzyme's active site through the template tunnel. In the presence of NTPs, product dsRNA exits through the product tunnel. (C) During transcription, the (+)RNA strand is displaced while φ6 RdRP polymerizes a new (+)RNA strand (green line) onto the (−)RNA template. Here, the dsRNA genome is first unwound, and the (+)RNA strand is displaced at the entrance to the template tunnel, allowing only the (−)RNA strand to enter the template tunnel. The dsRNA product exits through the product tunnel. The polarities of the RNA strands are indicated in the schematics.