Restriction-Modification Systems
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The Type I restriction and modification systems are divided
into four families (Type IA e.g.
EcoKI,
Type IB e.g. EcoAI, Type IC e.g.
EcoR124I and
Type ID e.g. StySBLI) based on
cross-hybridisation of genes and antibody cross- reactivity (Barcus et al., 1995;
Bickle, 1987). Although the
gene order varies between different families, in all cases the
genes are expressed from two promoters: PRes is
responsible for transcription of the hsdR gene and PMod
is responsible for transcription of the hsdM and hsdS
genes. The hsdM and hsdS genes overlap, usually
by one base pair, which may provide translational control of the
level of HsdS and HsdM. Such control at the level of translation
has been observed for a number of systems (e.g. TaqI -
Barany et al., 1992).
This control would imply that there should be more HsdM than HsdS
in the cell. In fact, this control was demonstrated for the
EcoKI
system when the level of subunits was analysed in minicells. In
addition, the HsdR subunit was found to be present in the lowest
concentration relative to HsdS and HsdM (Weiserova et al.,
1993). No control at the level of transcription has been observed
for any of these systems (Loenen
et al., 1987, our own observations;
Prakash-Cheng et al.,
1993). The individual subunits of family members can
complement for each other and as a consequence the DNA
specificity of one enzyme can be changed to that of another
enzyme by introduction of the appropriate HsdS subunit. Within
each family there are distinct regions
of the HsdS subunit in which amino acid identities are strongly
conserved. One such region lies about midway between the C- and N-
termini and is |
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known as the central
conserved region; while the
other region is at the C-terminus (