Restriction endonucleases will degrade invading double-stranded phage or plasmid DNA that
is not appropriately modified. However, under certain stress conditions (for
example, induction of the SOS response or heat shock) restriction endonucleases
can be temporarily inactivated [Kelleher and Raleigh 1994; Barcus and Murray
1995]. Furthermore, many phage and plasmids have mechanisms to avoid
degradation of their DNA. Such mechanisms are called "antirestriction systems".
Some examples of antirestriction systems are described below.
- "Loss" of restriction sites. Some phage have many fewer recognition
sites for certain restriction endonucleases than you would predict by random
chance. For example, the phages T3 and T7 lack the 5-bp EcoRII
recognition site CC(A or T)GG. This is thought to arise by selection for phage
with mutations that prevent cleavage by restriction endonucleases commonly
encountered in their hosts [Korona et al. 1993; Kruger et al. 1995].
- Modified bases. Some phage have modified bases that prevent
recognition by restriction endonucleases. For example, phages T2 and T4 have
hydroxylmethylcytosine instead of cytosine in their DNA [Carlson et al.
- Self-methylation. Some phage encode a methylase that can modify their
DNA so that it is not cleaved by certain restriction endonucleases. For
example, the E. coli phages T2, T4, and the Myxococcus phage Mx8
encode dAdenine methylases (dam) [Carlson et al. 1994; Magrini et al.
1997]. Certain plasmids also encode methylases that may provide protection
against restriction endonucleases [Ibanez et al. 1997].
- Activation of a host methylase. Some phage encode a protein that
stimulates the host's methylase to modify the phage DNA. For example, the
Lambda Ral ("restriction alleviation") protein enhances methylation by the HsdM
subunit of the EcoK and EcoB restriction systems [Court and
Oppenheim 1983]. Ral seems to act by stimulating the expression of the host
hsdS and hsdM genes [Loenen and Murray 1986].
- Degradation of host S-adensylmethionine. Some host restriction systems
only recognize modified DNA (e.g. the McrA and McrB systems in E. coli).
Phage T3 encodes a S-adensylmethionine hydrolase activity that degrades the
substrate required for methylation by host enzymes, thus avoiding modification
of the phage DNA and subsequent cleavage by modification-dependent host
endonucleases [Kruger et al. 1985].
- Inhibition of host restriction endonucleases. Many conjugal plasmids
produce antirestriction proteins (called Ard) that specifically inhibit Type I
restriction endonucleases. The Ard proteins have a motif that is very similar
to a motif found in the HsdS subunit, thus it is possible that the Ard proteins
prevent proper assembly of the restriction endonuclease complex by binding to
the other Hsd subunits [Belogurov and Delver 1995]. Phage T3 produces a protein
called Ocr which inhibits host methylases, resulting in resistance to
modification-dependent restriction systems [Kruger et al. 1985].
- DNA repair systems. Activity of certain phage genes may
allow repair of the double-stranded breaks produced by restriction
endonucleases. For example, the lambda Gam and Red proteins may repair the
cleaved DNA by recombination with another copy of the phage DNA [Salaj-Smic et
For a nice recent review see:
Wilkins B. 2002.Plasmid promiscuity: meeting the challenge of DNA immigration control.
Environ Microbiol. 4: 495-500.
- Belogurov, A., and E. Delver. 1995. A motif conserved among the type I
restriction-modification enzymes and antirestriction proteins: a possible basis
for mechanism of action of plasmid-encoded antirestriction functions. Nucleic
Acids Res. 23: 785-787.
- Barcus, V., and N. Murray. 1995. Barriers to recombination: restriction. In
S. Baumberg, J. Young, E. Wellington, and J. Saunders (eds.), Population
Genetics of Bacteria, pp. 31-58. University Press, Cambridge.
- Carlson, K., E. Raleigh, and S. Hattman. 1994. Restriction and modification.
In J. Karam (ed.), Molecular Biology of Bacteriophage T4, pp. 369-381. American
Society for Microbiology, Washington, D.C.
- Court, D., and A. Oppenheim. 1983. Phage Lambda's accessory genes. In R.
Hendrix, J. Roberts, F. Stahl, and R. Weisberg (eds.), Lambda II, pp. 251-277.
Cold Spring Harbor Laboratory, NY.
- Ibanez, M., I. Alvarez, J. Rodriguez-Pena, and R. Rotger. 1997. A ColE1-type
plasmid from Salmonella enteritidis encodes a DNA cytosine methyltransferase.
Gene 196: 145-158.
- Kelleher, J., and E. Raleigh . 1994. Response to UV damage by four
Escherichia coli K-12 restriction systems. J. Bacteriol. 176: 5888-5896.
- Korona, R., B. Korona, and B. Levine. 1993. Sensitivity of naturally occuring
coliphages to type I and type II restriction and modification. J. Gen.
Microbiol. 139: 1283-1290.
- Kruger, D., C. Schroeder, M. Reuter, I. Bogdarina, Y. Buryanov, T. Bickle.
1985. DNA methylation of bacterial viruses T3 and T7 by different DNA
methylases in Escherichia coli K12 cells. Eur. J. Biochem. 150:
- Kruger, D., D. Kupper, A. Meisel, M. Reuter, and C. Schroeder. 1995. The
significance of distance and orientation of restriction endonuclease
recognition sites in viral DNA genomes. FEMS Microbiol. Rev. 17: 177-184.
- Loenen, W., and N. Murray. 1986. Modification enhancement by the restriction
alleviation protein (Ral) of bacteriophage lambda. J. Mol. Biol. 190: 11-22.
- Magrini, V., D. Salmi, D. Thomas, S. Herbert, P. Hartzell, and P. Youderian.
1997. Temperate Myxococcus xanthus phage Mx8 encodes a DNA adenine
methylase, Mox. J. Bacteriol. 179: 4254-4263.
- Salaj-Smic, E., N. Marsic, Z. Trgovcevic, and R. Lloyd. 1997. Modulation of
EcoKI restriction in vivo: role of the lambda Gam protein and plasmid
metabolism. J. Bacteriol. 179: 1852-1856.
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