Mutational analysis of the direct tandem repeat sequences at the origin of DNA replication of porcine circovirus type 1

Mutational analysis was conducted to investigate the role of the nucleotide sequences flanking the stem-loop palindromic structure at the origin of DNA replication of porcine circovirus type 1 (PCV1) with respect to self-DNA replication and progeny virus generation. The results demonstrated that the A-rich sequence to the left of the palindrome is non-essential for virus replication. Although a set of four hexanucleotide (H) sequences to the right of the palindrome (organized in two tandem repeats: the proximal H1/H2 and the distal H3/H4) are binding sites for the viral Rep-associated proteins in vitro, only a proximal tandem (H/H or h-like/H) is essential for PCV1 DNA replication. In the presence of H1/H2, mutations engineered into H3/H4 were preserved in the progeny viruses. Mutations engineered into H1/H2 were invariably deleted so that the downstream H3/H4 was placed next to the palindrome. Viral genome with mutations engineered into both H1/H2 and H3/H4 underwent extensive nucleotide reorganization to yield progeny viruses containing either H3/H4, h-like/H4, or h-like/H3/H4 sequences.