M13 phage (Figure 12) based cloning vectors has been used to obtain ssDNA templates that are used for the dideoxy chain termination method of sequencing DNA. M13 bacteriophages can be considered true pioneer in the field of molecular cloning vectors.

M13 phage is 7.2 kb long bacteriophage designated as (+) strand, which is converted into a double stranded replicative form (RF) on infection of E.coli.

Figure 12. filamentous phage M13
Figure 12. filamentous phage M13

M13 phages can be purified for use in DNA sequencing or in vitro mutagenesis.

Cloning vectors based on Ml3RF DNA facilitate production of recombinant (+) strands that carry the sequence of one strand of a foreign DNA insert. These single-stranded DNA molecules serve as highly efficient templates determination of nucleotide sequence by dideoxy chain-termination method.

M13 was developed into a useful cloning vector by inserting the following elements into the genome

  • A gene for the lac repressor (lac I) protein to allow regulation of the lac promoter.
  • The operator-proximal region of the lac Z gene (to allow for a-complementation in a host with operator-proximal deletion of the lac Z gene).
  • A lac promoter upstream of the lac Z gene.
  • A polylinker (multiple cloning site) region inserted several codons into the lac Z gene.

They were named according to the specific polylinker region they contained. The vectors were typically constructed in pairs, with the polylinker regions in opposite orientations. Other M13 vectors such as M13mp9, M13mp10/11, and M13mp18/19 are also available with different polylinkers and restriction sites.

Phagemids that have replication origin of pUC plasmids and of the M13 bacteriophage have also been developed and are available under the trade name of LITMUS vectors.