FIG. 1.

The left end of the MVM genome. The left (3′) hairpin of the MVM genome contains both the upstream regulatory region of the P4 promoter and sequences that give rise to the left-end DNA replication origin. This origin, part of which is shown in expanded form, is a duplex copy of sequences from the top strand of the hairpin, which is formed in dimeric replication intermediates. The PIF half-sites (spaced ACGT motifs) overlapping a consensus ATF site are indicated and marked proximal (P) and distal (D) to denote their positions with respect to the NS1 binding site. The dinucleotide “bubble” sequence is a critical spacer element lying between the proximal ACGT and the NS1 binding site. The box which starts at the bubble sequence and extends rightwards toward the other end of the minimal origin indicates the sequences protected from DNase I digestion by NS1. Within this box are the NS1-binding motif, which overlaps the E box and NF-Y sites of the promoter, and the nick site, where NS1 initiates replication by nicking the DNA to liberate a base-paired 3′ nucleotide capable of priming DNA synthesis.