Type I Restriction and Modification Systems

Type I Families

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: P_Res is responsible for transcription of the hsdR gene and P_Mod 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 known as the central conserved region; while the other region is at the C-terminus (Cowan et al., 1989; Kannan et al., 1989; Murray et al., 1982). In addition, type IC HsdS subunits have a conserved region at the N-terminus. This region is normally present at the C-terminus of other Type I systems and is, therefore, part of a "split-repeat" (Kneale, 1994). Outside of these conserved regions are two highly variable regions responsible for DNA recognition, each of which recognises one-half of the split recognition sequences (Abadjieva et al., 1994; Cowan et al., 1989; Meister et al., 1993).