EARLY AND LATE PRE-MESSENGER-RNA PROCESSING OF BUDGERIGAR FLEDGLING DISEASE VIRUS .1. IDENTIFICATION OF VIRAL-RNA 5'-END AND 3'-END AND INTERNAL SPLICE JUNCTIONS

Budgerigar fledgling disease virus 1 (BFDV-1) is the first avian polyo mavirus to be identified, and it possesses uncommon structural and bio logical properties. Here we present an analysis of the processed viral RNAs in infected chicken embryo fibroblast cells. Two early and 18 la te BFDV-1 mRNAs were defined according to their 5' ends and internal s plice patterns. In the early region of the genome an incomplete splice reaction covering 195 nt is responsible for creating two mRNAs that c ould encode small t and large T antigens, which would be initiated fro m a hypothetical early promoter, P-E. The late mRNA 5' ends define two putative promoter regions (P-L1 and P-L2), 111 nt apart in the BFDV-1 genome non-coding region. The overall splicing pattern of the late mR NAs is further complicated by an alternative splice reaction of intron 2 (deletion of either 64 nt in intron 2a or of 256 nt in intron 2b) a nd a splice removing intron 3 (870 nt), resulting in deletion of most of the VP2-VP3 coding region. The positions of the late mRNA 5' ends a nd the splicing pattern indicate the existence of two open reading fra mes, putatively encoding two pairs of agnoproteins, in the 5' region o f several late mRNAs. These mRNAs appear to be bicistronic and to enco de one of the agnoproteins together with one of the viral coat protein s.