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Journal of Bacteriology, March 2008, p. 2039-2049, Vol. 190, No. 6
0021-9193/08/$08.00+0     doi:10.1128/JB.01319-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Maintenance Forced by a Restriction-Modification System Can Be Modulated by a Region in Its Modification Enzyme Not Essential for Methyltransferase Activity{triangledown}

Satona Ohno,{dagger},{ddagger} Naofumi Handa,{dagger} Miki Watanabe-Matsui,§ Noriko Takahashi, and Ichizo Kobayashi*

Department of Medical Genome Sciences, Graduate School of Frontier Science and Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan

Received 14 August 2007/ Accepted 2 January 2008

Several type II restriction-modification gene complexes can force their maintenance on their host bacteria by killing cells that have lost them in a process called postsegregational killing or genetic addiction. It is likely to proceed by dilution of the modification enzyme molecule during rounds of cell division following the gene loss, which exposes unmethylated recognition sites on the newly replicated chromosomes to lethal attack by the remaining restriction enzyme molecules. This process is in apparent contrast to the process of the classical types of postsegregational killing systems, in which built-in metabolic instability of the antitoxin allows release of the toxin for lethal action after the gene loss. In the present study, we characterize a mutant form of the EcoRII gene complex that shows stronger capacity in such maintenance. This phenotype is conferred by an L80P amino acid substitution (T239C nucleotide substitution) mutation in the modification enzyme. This mutant enzyme showed decreased DNA methyltransferase activity at a higher temperature in vivo and in vitro than the nonmutated enzyme, although a deletion mutant lacking the N-terminal 83 amino acids did not lose activity at either of the temperatures tested. Under a condition of inhibited protein synthesis, the activity of the L80P mutant was completely lost at a high temperature. In parallel, the L80P mutant protein disappeared more rapidly than the wild-type protein. These results demonstrate that the capability of a restriction-modification system in forcing maintenance on its host can be modulated by a region of its antitoxin, the modification enzyme, as in the classical postsegregational killing systems.

* Corresponding author. Mailing address: Department of Medical Genome Sciences, Graduate School of Frontier Science and Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan. Phone: 81 3 5449 5326. Fax: 81 3 5449 5422. E-mail: ikobaya{at}ims.u-tokyo.ac.jp

{triangledown} Published ahead of print on 11 January 2008.

{dagger} S.O. and N.H. contributed equally to this work.

{ddagger} Present address: Laboratory of Gene Expression and Regulation, Institute of Medical Science, University of Tokyo, Tokyo, Japan.

§ Present address: Department of Biochemistry, Graduate School of Medicine, Tohoku University, Sendai, Japan.

Journal of Bacteriology, March 2008, p. 2039-2049, Vol. 190, No. 6
0021-9193/08/$08.00+0     doi:10.1128/JB.01319-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Handa, N., Ichige, A., Kobayashi, I. (2009). Contribution of RecFOR machinery of homologous recombination to cell survival after loss of a restriction-modification gene complex. Microbiology 155: 2320-2332 [Abstract] [Full Text]  


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