Viruses are the only organisms known to store their genetic information solely in the form of RNA, and have thus evolved unique machinery to replicate an RNA genome and initiate viral gene expression in the infected cell. The large polymerase protein (L) of negative-strand (NS) RNA viruses is a particularly intriguing model for viral replication, where all of the enzymatic activities required for mRNA transcription, RNA modification, and genomic RNA replication are contained within a single polypeptide. Whereas the host cell requires a suite of enzymes to accomplish these tasks, L alone is the catalytic engine driving NS RNA viral replication. Here we demonstrate purification of functional L protein from Machupo virus (MACV) and reconstitute arenavirus RNA synthesis initiation and gene expression regulation in vitro using purified recombinant components. Through single-molecule electron microscopy analysis of MACV L, we provide the first structural information of viral L proteins. Comparative analysis with nonsegmented NS RNA viral L proteins reveals how the various enzymatic domains are arranged into a conserved architecture shared by both polymerases. Our in vitro RNA synthesis data defines the basis of arenavirus sequence-specific polymerase recruitment and how inter-termini interactions regulate template recognition. Moreover, we discover a new role for the arenaviral matrix protein in regulating viral RNA synthesis by locking a polymerase-template complex. The inhibitory matrix-L-RNA assembly functionally links transcription regulation and polymerase packaging, and reveals a mechanism for NS RNA viruses to ensure polymerase incorporation during virion maturation. Reconstitution of RNA synthesis in vitro establishes a new framework to understand the arenaviral polymerase complex, and our structural and biochemical experiments provide a basis for mechanistic analysis of the NS RNA viral replication machinery.

Reverse genetics is a technology that allows the production of a virus from its complementary DNA (cDNA). It is a powerful tool for analyzing viral genes, the development of novel vaccines, and gene delivery vectors. The standard reverse genetics protocols are laborious, time-consuming, and inefficient for negative-strand RNA viruses. A new reverse genetics platform was established, which increases the recovery efficiency of the measles virus (MV) in human 293-3-46 cells. The novel features compared with the standard system involving 293-3-46 cells comprise (a) dual promoters containing the RNA polymerase II promoter (CMV) and the bacteriophage T7 promoter placed in uni-direction on the same plasmid to enhance RNA transcription; (b) three G nucleotides added just after the T7 promoter to increase the T7 RNA polymerase activity; and (c) two ribozymes, the hairpin hammerhead ribozyme (HHRz), and the hepatitis delta virus ribozyme (HDVrz), were used to cleavage the exact termini of the antigenome RNA. Full-length antigenome cDNA of MV of the wild type IC323 strain or the vaccine AIK-C strain was inserted into the plasmid backbone. Both virus strains were easily rescued from their respective cloned cDNA. The rescue efficiency increased up to 80% compared with the use of the standard T7 rescue system. We assume that this system might be helpful in the rescue of other human mononegavirales.