A 5′-proximal Stem-loop Structure of 5′ Untranslated Region of Porcine Reproductive and Respiratory Syndrome Virus Genome Is Key for Virus Replication

Background: It has been well documented that the 5' untranslated region (5' UTR) of many positive-stranded RNA viruses contain key cis-acting regulatory sequences, as well as high-order structural elements. Little is known for such regulatory elements controlling porcine arterivirus replication. We investigated the roles of a conserved stem-loop 2 (SL2) that resides in the 5' UTR of the genome of a type II porcine reproductive and respiratory syndrome virus (PRRSV). Results: We provided genetic evidences demonstrating that 1) the SL2 in type II PRRSV 5' UTR, N-SL2, could be structurally and functionally substituted by its counterpart in type I PRRSV, E-SL2; 2) the functionality of N-SL2 was dependent upon the G-C rich stem structure, while the ternary-loop size was irrelevant to RNA synthesis; 3) serial deletions showed that the stem integrity of N-SL2 was crucial for subgenomic mRNA synthesis; and 4) when extensive base-pairs in the stem region was deleted, an alternative N-SL2-like structure with different sequence was utilized for virus replication. Conclusion: Taken together, we concluded that the phylogenetically conserved SL2 in the 5' UTR was crucial for PRRSV virus replication, subgenomic mRNA synthesis in particular.