DNA polymerases are unable to copy unprimed DNA templates. Various mechanisms have therefore evolved in different biological systems to provide the template strand to be replicated with a free 3Ј-hydroxyl end that can be extended. These mechanisms include RNA priming in the case of pro-and eucaryotes as well as some viruses (1), priming through a DNA-bound protein in the case of adenovirus, some bacteriophages and various linear plasmids (2), as well as self-priming at hairpins created by the folding-back of terminal palindromic sequences (3). Palindromic termini are present in poxvirus (4, 5) and parvovirus (6, 7) telomeres, paramecium mitochondrial DNA (8), and tetrahymena rDNA (9). The terminal palindromes of pox-and parvovirus genomes play an essential role in distinct steps of viral DNA replication, including the priming of concatemeric intermediates formation and their subsequent resolution into monomeric daughter molecules (10 -15). Mutational analyses indicated the existence of specific sequence elements within the core of poxvirus DNA palindromes, which are required for the formation of hairpins (16). Given that the transition of palindromic DNA from the duplex into the hairpin configuration requires the overcome of a high energetic barrier, factor(s) interacting with specific DNA elements may be necessary for this structural transition.Minute virus of mice (MVM), 1 a prototype member of the autonomously replicating parvoviridae (17), makes use of a hairpin-priming mechanism to replicate its linear, 5149-nucleotide (nt) (18) single-stranded (ss) DNA genome. MVM DNA replication starts with complementary strand synthesis primed at the genomic left-hand (3Ј-terminal) hairpin, producing a double-stranded (ds) replicative form (RF) DNA (19,20). As demonstrated recently in vitro for the majority of processed DNA molecules, complementary strand synthesis stops when reaching the folded-back right-hand (5Ј-terminal) hairpin, and is followed by ligation of the newly synthesized and parental strands. This results in a molecule covalently closed at both ends (cRF) (14). Such closed forms were also detected in parvovirus-infected cells (21,22). Further processing of cRF DNA in vitro requires the MVM nonstructural protein NS1 (14, 15). When added as a purified polypeptide expressed from baculovirus vectors, NS1 was found to nick the MVM complementary strand 21 nt inboard of the folded-back genomic 5Ј terminus, followed by initiation of strand-displacement synthesis and copying of the hairpin to yield a molecule that is extended at its right end (15). Rearrangement of the copied right-hand palindrome into hairpin structures (formation of the so-called rabbit-eared configuration) provides a primer for reinitiation of replication in a strand-displacement manner leading to the formation of concatemeric molecules, in particular dimerlength RF DNA (14,23).Restoration of hairpin structures at the duplex right-hand telomere of MVM dsDNA templates (14,15,24,25) and formation of dimer-length RF DNA (14) were recently achieved ...