Analysis of transcription of Porcine circovirus type 1

Porcine circovirus type 1 (PCV1) contains two major open reading frames encoding the replication initiator proteins, Rep and Rep', and the structural protein, Cap. The promoters of these two genes (P(cap) and P(rep)) have been mapped. P(cap) is located within the rep open reading frame (nt 1328-1252). P(rep) has been mapped to the intergenic region immediately upstream of the rep gene (nt 640-796) and overlaps the origin of replication of PCV1. Although binding of both rep gene products to a fragment containing P(rep) and the overlapping origin of replication has been reported, only the full-length Rep protein repressed P(rep), while the spliced isoform Rep' did not. P(rep) repression is mediated by binding of the Rep protein to the two inner hexamers, H1 and H2, located in the origin of PCV1, whereas binding of Rep to hexamers H3 and H4 was not necessary. Use of Rep mutants indicated that the conserved rolling-circle replication domain II as well as the P loop are essential for repression of P(rep). In contrast to P(rep), transcription of P(cap) was not influenced by viral proteins. Additionally, the ratio of the rep and rep' transcripts was analysed. Twelve hours after transfection of PK15 cells with an infectious clone of PCV1, similar amounts of both transcripts were detected, but later the amount of the two transcripts varied, indicating a balanced expression of the two rep transcripts.