Restriction-modification system

Burrus, V., C. Bontemps, B. Decaris, and G. Guedon. 2001. Characterization of a novel type II restriction-modification system, Sth368I, encoded by the integrative element ICEStl of Streptococcus thermophilus CNRZ368. Appl Environ Microbiol 67(4) 1522-8. 

These enzymes are isolated from bacteria, their natural source. There are many different restriction endonucleases isolated from a variety of bacteria that are now readily available commercially. In bacteria they act as part of a restriction/modification system that protects the bacteria from infection by DNA viruses. 

Type II restriction-modification systems differ from their type I and type III counterparts in that the endonuclease and DNA methylase activities are conducted by two separate enzymes (not a single multisubunit complex). The restriction endonuclease cleaves both strands of the DNA duplex within a defined recognition sequence, while the companion DNA methylase methylates a specific base within the same recognition sequence. In contrast... 

Figure V-15 The DNA methylase component of the restriction modification system methylates a specific cytosine or adenine residue, making it incapable of being acted on by the companion restriction endonuclease.

Fig. 13.1. Biological function of restriction/modification systems RM systems recognize and act on short palindromic DNA sequences. While the host genome is protected by the methyltransferase activity of the system, invading phage DNA is cleaved by the endonuclease activity.

A. Jeltsch and A. Pingoud. 1996. Horizontal gene transfer contributes to the wide distribution and evolution of type II restriction-modification systems J. Mol. Evol. 42 91-96. (PubMed)... 

Type I DNA restriction/modification systems have been found in many strains of Escherichia coli and Salmonella enterica (Bickle Kruger,... 

Restriction-modification system. A pair of enzymes found in most bacteria (but not eukaryotic cells). The restriction enzyme recognizes a certain sequence in duplex DNA and makes one cut in each unmodified DNA strand at or near the recognition sequence. The modification enzyme methylates (or modifies) the same sequence, thus protecting it from the action of the restriction enzyme. 

Horizontal gene transfer may be a common event. For example, genes that inactivate antibiotics arc often transferred, leading to the transmission of antibiotic resistance from one species to another. For restriction-modification systems, protection against viral infections may have favored horizontal gene transfer. 

At present, three different types of restriction-modification systems are known-types I, II, and III. 

Write the reaction and explain the biological role of methylases (DNA methyltransferases) in restriction-modification systems. 

Restriction. The bacterial restriction modification system consists of DNA restriction endonuclease (Pingoud, 2004) and a matched modification enzyme (methylase, i.e. methyltransferase). The restriction endonucleases recognize specific sequences within dsDNA on which the hydrolysis takes place. Three types of restriction enzymes (Table 13.4) have been identified (Yuan, 1981). 

Grogan, D.W. (2003) Cytosine methylation by the Sual restriction-modification system implications for genetic fidelity in a hyperthermophilic archaeon. /. Bacterial, 185, 4657-4661. 

Two types of restriction-modification system have been found in bacteria. In type I systems, the methy-lase and R. e. are both associated with a complex containing three different polypeptide chains an a-chain with R.e activity, a P-chain with methylase activity and a y-chain with the recognition site for the DNA sequence. Type I systems require S-adenosyl-t-me-thionine and ATP for both R.e. and methylase activities they are less specific, and cleavage sites may be random and far removed (1,000 base pairs) from the 5 side of the recognition site. In type II systems, me-thylases and R.e. are separate, 5-adenosyl-L-methio-... 

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