c cis-Acting elements in the viral genome RNA (vRNA) are essential for the translation, replication, and/or encapsidation of RNA viruses. In this study, a novel conserved cis-acting element was identified in the capsid-coding region of mosquito-borne flavivirus. The downstream of 5= cyclization sequence (5=CS) pseudoknot (DCS-PK) element has a three-stem pseudoknot structure, as demonstrated by structure prediction and biochemical analysis. Using dengue virus as a model, we show that DCS-PK enhances vRNA replication and that its function depends on its secondary structure and specific primary sequence. Mutagenesis revealed that the highly conserved stem 1 and loop 2, which are involved in potential loop-helix interactions, are crucial for DCS-PK function. A predicted loop 1-stem 3 base triple interaction is important for the structural stability and function of DCS-PK. Moreover, the function of DCS-PK depends on its position relative to the 5=CS, and the presence of DCS-PK facilitates the formation of 5=-3= RNA complexes. Taken together, our results reveal that the cis-acting element DCS-PK enhances vRNA replication by regulating genome cyclization, and DCS-PK might interplay with other cis-acting elements to form a functional vRNA cyclization domain, thus playing critical roles during the flavivirus life cycle and evolution.
T he flavivirus genus contains numerous important agents of human infectious diseases, including dengue virus (DENV), West Nile virus (WNV), Japanese encephalitis virus (JEV), yellow fever virus (YFV), and tick-borne encephalitis virus (TBEV). Human infection by flaviviruses can result in symptoms ranging from mild fever to severe encephalitis and hemorrhagic fever. Due to lack of effective vaccines and specific medicines against many flaviviruses, they pose a significant threat to human health around the world.Flaviviruses are enveloped RNA viruses with single-stranded, positive-sense genomes. Their 5= capped viral genome RNA (vRNA) is approximately 10 to 11 kb and contains a single open reading frame (ORF) flanked by 5= and 3= untranslated regions (UTRs). The ORF encodes a polyprotein with more than 3,000 residues, which is cotranslationally and/or posttranslationally proteolytically processed into three structural proteins (capsid, pre-membrane/membrane, and envelope) and seven nonstructural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5) by viral and cellular proteases. NS3 and NS5 perform essential enzyme activities required for vRNA replication. The C-terminal domain of NS3 has helicase/NTPase (1) and RNA triphosphatase activities (2, 3). The N-terminal domain of NS5 encodes guanylyltransferase (4) and methyltransferase activities (5, 6), whereas the C-terminal domain of NS5 has RNA-dependent RNA polymerase (RdRp) activity. These enzyme activities are involved in vRNA synthesis, 5= capping, and internal adenosine methylation (7).The 5=UTR, 3=UTR, and capsid-coding sequence are involved in vRNA replication, translation, and perhaps encapsidation, and many cis-acting elements have ...