|
|
|
|
New R-M systemsThe classic Type I R-M systems were found in enteric bacteria such as Escherichia coli and Salmonella typhimurium. However, more recently Type I R-M systems have been found in other bacteria including Citrobacter freundi and Mycoplasma pulmonis (Dybvig et al., 1994). In addition many new systems have been identified in a collection of hospital E. coli strains (the ECOR collection) (Barcus et al., 1995). These new systems have helped confirm that Type I R-M systems are capable of rapid evolution of DNA specificity by means of recombination, leading to swapping or exchange of "target recognition domains". Two independent R-M systems have been identified in Mycoplasma pulmonis, and both have been found to be capable of switching DNA specificity by means of a DNA inversion. This switch is genetically linked to a switch in the production of surface antigens. Recent genome sequencing programs have shown that Type I R-M systems are much more widespread than originally thought (Bult et al., 1996; Fraser, et al., 1995). This has encouraged a debate about the distribution and function of restriction modification systems in general (Raleigh & Brooks, 1998). A few members of a novel R-M system were recently described at the 4th New England BioLabs Workshop on Biological DNA Modification. These systems were grouped as Type 1½ and are typified by the AhdI R-M system (Marks et al 2003, 2004). The most interesting feature of these systems is their arrangement of the hsdM and hsdS genes. The hsdM gene is fused to a deleted version of the hsdS gene to produce a single subunit capable of DNA binding and methylation (SAM-binding). The recognition sequence is symmetrical and resembles that of the deleted HsdS(R124)D50 system. Therefore, these R-M systems appear to be part of a continuum of R-M families in which simple TRDs, capable of recognising short 3-4bp sequences, can dimerise to form symmetrical Type I R-M systems. Evolution of other TRDs has led through recombination to attachment of a second TRD to the initial TRD to yield conventional Type I R-M systems. In addition, gene fusion of the simple, single TRDs to hsdM has led to the formation of Type 1½ systems. One can also imagine that Type III R-M systems are also part of this continuum. |
|
• © University of
Portsmouth |
