a, Superposition of LASV (dark) and MACV (light) L structures. The four major domains with different conformations are highlighted, including the endonuclease (blue), α-ribbon (yellow), pendant (pink) and clamp (green) domains. The rest of the main body is coloured in grey. The pendant domain of LASV is modelled by superimposing the α-ribbon and pendant of MACV L with the α-ribbon helices of LASV and is highlighted by a dotted oval. The CTD is not resolved in both structures and is represented with a black dotted oval. The predicted template entrance is indicated by a black arrow, which is closed by the α-ribbon and pendant in MACV L. b, Schematic model of the different conformational states of LASV and MACV L proteins. The domains of LASV L are represented by solid colours, and those of MACV L are shown with dotted outlines. c, Model of replicating RdRp based on the structures of LASV L and the poliovirus elongation complex30. The catalytic site is indicated by a red asterisk. The 3′-vRNA (black) and product (blue) strands are separated by the lid domain. The NTP entrance and template or product exits are indicated by arrows. d, Inside tunnels within the LASV L structure for RNA synthesis, including the template entrance, the NTP entrance and the template (black) or product (blue) exits. The catalytic site is indicated by a red asterisk.