Avian polyomavirus expression patterns of bicistronic late mRNAs.

Avian polyomavirus (APV, previously called budgerigar fledgling disease virus, BFDV-1) differs from other polyoma viruses by a complex arrangement within its late genes: p(L1)-promoted agno-genes 1a and 1b overlap p(L2)-promoted agno-genes 2a and 2b in different reading frames, and are located upstream of standard late VP genes. As a minimal set, agno-gene 1a plus VP1, VP2, VP3 support effective viral propagation in cell culture, together with the origin-region and T-antigen section. All viral late mRNAs are (at least) bicistronic, and have an agno-gene located upstream of VP2/VP3 or VP1. Kinetic experiments show that proximal agno-protein 1a translation is dominant until about 20 h post infection, when high-level distal VP protein synthesis takes over. Ribosomal leaky scanning appears to prevail within the 5' region of APV late mRNAs, but cannot explain the observed consecutive expression from bicistronic mRNAs. Instead, accumulated agno-protein 1a may be involved in that shift of translation.