The discovery of the phenomenon of host-controlled
restriction-modification in bacteriophages led to the isolation
of restriction endonucleases, which later became
universal tools for the specific manipulation of DNA.
Bertani and Weigle [16] uncovered that most, but not all,
λ bacteriophage particles grown on E. coli K-12 were
unable to multiply in a strain of E. coli that carries the
prophage P1 (E. coli K-12(P1)). The rare λ phage particles
that succeeded to multiply were subsequently able to
propagate at full efficiency in E. coli K-12(P1). This phenomenon
was explained by two functions of the prophage
P1: first, that it restricted the propagation of λ phage in
E. coli K-12(P1) and, second, that it modified the few survivors
and so allowed them to propagate in this strain.
Restriction is a general phenomenon due to enzymes
(restriction endonucleases) that destroy ‘foreign DNA’ by
cutting it up. The cutting occurs at sites that depend on
the specificity of the endonuclease present in particular
cells for particular base sequences in DNA. Restriction
may be seen as a kind of defence mechanism against
extraneous genetic material. Modification is due to the
methylation of DNA by methylase enzymes to protect it
from the action of restriction endonucleases. The subject
has been reviewed by Wilson and Murray [17].