Mutational mechanisms by which an inactive replication origin of bacteriophage M13 is turned on are similar to mechanisms of activation of ras proto-oncogenes.

M13 viral strand synthesis is initiated by nicking of the viral strand of the duplex replicative form by the M13 gene II initiator protein at a specific site within a sequence of about 40 base pairs having dyad symmetry. Efficient replication of the M13 viral strand also requires the presence of an adjacent sequence of ca. 100 base pairs. Together these sequences constitute the minimal origin for M13 viral strand synthesis. A pBR322 derivative having a 182-base-pair insert of M13 DNA contains a functional M13 viral strand origin and, when provided with M13 gene functions in trans, replicates under conditions nonpermissive for the parent plasmid. Chimeric plasmids containing deletions within the sequence flanking the viral strand origin are unable to replicate under these conditions. We isolated spontaneous mutants of M13 based on their ability to activate replication of such plasmids. The mutations found in these strains, as well as several produced by oligonucleotide-directed mutagenesis, all result in the substitution of any of at least four different amino acids for a specific glycine residue near the amino-terminal end of the initiator protein. Other studies have shown that overproduction of the wild-type initiator protein also restores replication. These alternate mechanisms are discussed in terms of their striking similarity to the mechanisms of activation of the ras proto-oncogenes which can be activated either by increased expression of the wild-type protein or by substitution of any of several amino acids for a glycine residue near the amino terminus.