Classical swine fever virus NS5A regulates viral RNA replication through binding to NS5B and 3′UTR

In this report, classical swine fever virus (CSFV) NS5A inhibit viral RNA replication when its concentration reached and surpassed the level of NS5B. Three amino acid fragments of CSFV NS5A, 137–172, 224–268 and 390–414 individually were shown to be essential to NS5B binding. The former two fragments were independently necessary for regulation of viral RNA replication and correlated with NS5B and 3′UTR binding activity. We also found that amino acids W143, V145, P227, T246, P257, K399, T401, E406 and L413 of CSFV NS5A were essential to NS5B binding activity. Furthermore, these amino acids were shown to be necessary for viral RNA replication and infection and conserved in NS5A proteins of CSFV, BDV, BVDV and HCV. These results indicated that NS5A may regulate viral RNA replication by binding to NS5B and 3′UTR. NS5A can still regulate viral RNA synthesis through binding to 3′UTR when binding to NS5B is not available.