INITIATION OF DNA-REPLICATION AT PALINDROMIC TELOMERES IS MEDIATED BYA DUPLEX-TO-HAIRPIN TRANSITION INDUCED BY THE MINUTE VIRUS OF MICE NONSTRUCTURAL PROTEIN NS1

The linear single-stranded DNA genome of the minute virus of mice (MVM ) is replicated via a double-stranded replicative form (RF) intermedia te, Amplification of this RF is initiated by the folding-back of palin dromic sequences serving as primers for strand-displacement synthesis and formation of dimeric RF DNA. Using an in vitro replication assay a nd a cloned MVM DNA template, we observed hairpin-primed DNA replicati on at both MVM DNA termini, with a bias toward right-end initiation. I nitiation of DNA replication is favored by nuclear components of A9 ce ll extract and highly stimulated by the MVM nonstructural protein NS1. Hairpin-primed DNA replication is also observed in the presence of NS 1 and the Klenow fragment of the Escherichia coil DNA polymerase I. Ad dition of ATP gamma S (adenosine 5'-O-(thio-triphosphate)) blocks the initiation of DNA replication but not the extension of pre-existing ha irpin primers formed in the presence of NS1 only, The NS1-mediated unw inding of the right-end palindrome may account for the recently report ed capacity of NS1 for driving dimer RF synthesis in vitro.