The genome of the filamentous bacteriophage M13 comprises a single-stranded circular DNA molecule, designated the (+) strand, which is converted into a double-stranded replicative form (RF) on infection ofEscherichia coli. Cloning vectors based on M13 RF DNA facilitate the production of recombinant (+) strands that carry the sequence of one strand of a foreign DNA insert (1 ,2 ). These single-stranded DNA molecules serve as highly efficient templates for nucleotide sequence determination by the dideoxy chain-termination method (3 ).
