Distinct 5′ UTR Requirements for Translation of the Bicistronic X/P mRNA Among Avian Orthobornaviruses

Orthobornaviruses express X and the phosphoprotein (P) from a bicistronic X/P mRNA, and these proteins regulate polymerase activity. In mammalian orthobornaviruses, the 5' untranslated region (5' UTR) of the X/P mRNA controls the translational balance between X and P and thereby promotes efficient replication. Avian bornaviruses (ABVs) belong to two clades, clade-2 and -3, that differ in the structure and length of the 5' UTR of the X/P mRNA. However, the functional consequences of these differences remain unclear. Using reverse genetics, we generated chimeric viruses by reciprocally exchanging the 5' UTR of the X/P mRNA among clade 2 parrot bornavirus 5 (PaBV-5) and aquatic bird bornavirus 1 (ABBV-1) and clade 3 PaBV-4. In PaBV-5, a long 5' UTR with a stem-loop and an upstream ORF was required to maintain the X-to-P translational balance. On the other hand, replacing the 5' UTR of the X/P mRNA from PaBV-5 with that from PaBV-4 reduced X expression and markedly impaired viral growth. However, PaBV-4 tolerated the 5' UTR of the X/P mRNA from PaBV-5 without detectable effects on translation or replication, which suggests that translation of PaBV-4 X/P mRNA does not depend on the origin of the 5' UTR. Furthermore, ABBV-1 replicated efficiently with the 5' UTR of the X/P mRNA from PaBV-5 but was strongly attenuated with that from PaBV-4. Taken together, these results demonstrate a clade-dependent requirement for the 5' UTR for translation of the X/P mRNA and provide novel insights into the evolution of translational control in orthobornaviruses.