The second reason these methylases are of interest is that the modification state of plasmid DNA can affect the frequency of transformation in special situations. Transformation efficiency will be reduced when 1) Dam-modified plasmid DNA is introduced into Dam- E. coli (one might do this in order to prepare DNA for digestion with BclI (NEB #R0160), for example); and 2) Dam or Dcm-modified DNA is introduced into certain other bacterial species. In the first instance, it has been shown that replication initiation is suppressed when plasmid DNA is hemimethylated at Dam sites (4). Dam-modified plasmids therefore replicate once in Dam-cells, and are unable to replicate again. The second phenomenon, poor transformation into other bacterial species, has not been comprehensively characterized, but may be due to modification-dependent restriction systems (5-8) analogous to the Mcr and Mrr systems found in E. coli. By using Dam- Dcm- strains to grow shuttle vectors, the modification-dependent restriction found in such bacteria can be avoided.