*Author for correspondence. Present address: Department of Biology, Emory University, Atlanta, GA 30322, USA. Tel. (404) 727 2826; fax (404) 727 2880; email biobrl@emuvml.cc.emory.edu.
Protection against lethal infections by bacteriophage may seem the most likely role of restriction-modification (R-M) systems in bacteria and the reason for their evolution. There are, however, phenomena which question this phage-mediated selection hypothesis for the maintenance of extant R-M systems. Most prominent among these are the mechanisms phage have to avoid or otherwise limit the effects of the restriction endonucleases produced by their host bacteria. To evaluate the importance of these antirestriction mechanisms in Escherichia coli, we have examined the sensitivity of coliphage from natural and laboratory sources to a series of type I and II R-M systems. The results of our study indicate that, in vivo, restriction endonucleases have no effect on a substantial fraction of naturally occurring coliphage. The absence of restriction sites appears to be the most common reason why these phage are unaffected by type II restriction endonucleases, but other antirestriction mechanisms also operate. On the other hand, the frequency of naturally occurring coliphage sensitive to restriction appears sufficiently great for phage-mediated selection to be a viable hypothesis for the maintenance of R-M in E. coli and its accessory elements.
BickleT.A.1987; DNA Restriction and modification systems. In: Escherichia coli and Salmonella typhimurium Cellular and Molecular Biology pp. 692–696NeidhardtF.C.
Edited by Washington DC: American Society of Microbiology.;
KrugerD.H.,
BickleT.A.1983; Bacteriophage survival: multiple mechanisms for avoiding deoxyribonucleic acid restriction systems of their hosts. Microbiological Reviews 47:345–360
LevinB.R.1986; Restriction-modification and the maintenance of genetic diversity in bacterial populations. Conference on Evolutionary Processes and Theory pp. 669–688NevoE.,
KarlinS.
Edited by New York: Academic Press.;
LevinB.R.,
StewartF.M.,
ChaoL.1977; Resource limited growth, competition and predation. A model with some experimental studies with bacteria and bacteriophage. American Naturalist 111:3–24
SharpP.M.1986; Molecular evolution of bacteriophage: evidence of selection against the recognition sites of host retriction enzymes. Molecular Biology and Evolution 3:75–83