biostudies-literatureUnknown70(2)Wu MCThis study was sypported by Japan Society for the Promotion of Science (JSPS) KAKENHI, Grant Numbers: JP20H05682 (K.T.), JP24K22131 (K.T.), JPJSCCA20240006 (A.M.), 25H01316 (A.M.), JP23K14083 (T.K.), JP25K18813 (T.K.), and 23K14528 (M.S.); Japan Agency for Medical Research and Development (AMED), Grant Number: JP23bm1223017h0001 (K.T.); Kaketsuken Research Grant (K.T.); The Joint Usage/Research Center Program on Institute for Life and Medical Sciences (LiMe), Kyoto University (K.T.); and The LiMe Grant 2024 and 2025, Kyoto University (T.K.).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 reqMicrobiology and immunology80-90https://www.ebi.ac.uk/biostudies/studies/S-EPMC12868937biostudies-literatureDistinct 5' UTR Requirements for Translation of the Bicistronic X/P mRNA Among Avian Orthobornaviruses.PMC12868937Tomonaga KWu MCLeacy AKomorizono RSusta LMakino ASakai MKanda TfalseDistinct 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 req2026-01-01T00:00:00Z2026 Feb2026-07-15T12:03:24.742Z2026-07-04T03:12:44.557ZS-EPMC128689374135532510.1111/1348-0421.70031