Figure 1.

The PAM concept. (A) The conventional method for the transformation of bacteria. The introduced shuttle vector is degraded by a restriction enzyme of the target bacterium. A small amount of vector survives and replicates in the target bacterium. (B) A PAM plasmid carries all the modification methylase genes of the target host. A shuttle vector plasmid is introduced to E. coli host, which had harboured the PAM plasmid (PAM host). The shuttle vector is methylated by the modification enzyme encoded by the genes on the PAM plasmid in the E. coli host. The shuttle vector is then extracted and introduced into the target host by electroporation. The shuttle vector is resistant against restriction enzymes and yields higher transformation efficiency. (C) The R–M system has a complicated structure, such as a gene cluster that includes subunits or unknown accessory genes. Alternatively, the PAM plasmid, containing a modification gene and also unknown parts, could be introduced into the transformant harbouring a shuttle vector. The restriction enzyme acts, but some copies of the plasmid could survive in the PAM host. The plasmid is then extracted and introduced the target bacterium.