Bunyamwera Bunyavirus RNA Synthesis Requires Cooperation of 3′- and 5′-Terminal Sequences

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Abstract
Bunyamwera virus (BUNV) is the prototype of both the Orthobunyavirus genus and the Bunyaviridae family of segmented negative-sense RNA viruses. The tripartite BUNV genome consists of small (S), medium (M), and large (L) segments that are each transcribed to yield a single mRNA and are replicated to generate an antigenome that acts as a template for synthesis of further genomic strands. As for all negative-sense RNA viruses, the 3′- and 5′-terminal nontranslated regions (NTRs) of the BUNV S, M, and L segments exhibit nucleotide complementarity and, except for one conserved U-G pairing, this complementarity extends for 15, 18, and 19 nucleotides, respectively. We investigated whether the complementarity of 3′ and 5′ NTRs reflected a functional requirement for terminal cooperation to promote BUNV RNA synthesis or, alternatively, was a consequence of genomic and antigenomic NTRs having similar functions requiring sequence conservation. We show that cooperation between 3′- and 5′-NTR sequences is required for BUNV RNA synthesis, and our results suggest that this cooperation is due to nucleotide complementarity allowing 3′ and 5′ NTRs to associate through base-pairing interactions. To examine the importance of complementarity in promoting BUNV RNA synthesis, we utilized a competitive replication assay able to examine the replication ability of all possible combinations of interacting nucleotides within a defined region of BUNV 3′ and 5′ NTRs. We show here that maximal RNA replication was signaled when sequences exhibiting perfect complementarity within 3′ and 5′ NTRs were selected.

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