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

Abstract

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 restrictionmodification 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.

Evolutionary Genomics of Defense Systems in Archaea and Bacteria

“…Furthermore, the RM and especially TA modules often reside on plasmids and thus employ a piggyback mode of dissemination. Most importantly, although these defense modules lack means for active replication and/or transposition, they do possess the addiction mechanisms that ensure their own persistence (66, 122). …”

Coupling immunity and programmed cell suicide in prokaryotes: Life‐or‐death choices

“…From a complementary perspective, the persistence of the dedicated suicide systems as well as at least some immune systems, such as RM, has to do with the fact that TA and RM modules possess features of selfish genetic elements, or more specifically, make the host cells addicted to these modules by killing cells that purge them .…”

Cytosine DNA methylation influences drug resistance inEscherichia colithrough increasedsugEexpression

“…In the case of Dcm, Dcm-dependent methylation of phage DNA increases phage infection frequencies in cells that harbor a restriction enzyme that cuts at the Dcm recognition site (Hattman et al, 1973). Dcm also enhances the loss of plasmids with restriction enzymes that cut at 5′CCWGG3′ sites and protects cells against postsegregational killing (Takahashi et al, 2002;Ohno et al, 2008). However, Dcm is often present in cells that do not harbor a restriction enzyme that cuts the same site and is therefore considered an orphan methyltransferase that may have additional functions.…”