Abstract
Restriction–modification (R–M) systems are widespread among prokaryotes and, depending on their type, may be viewed as selfish genetic elements that persist as toxin–antitoxin modules, or as cellular defense systems against phage infection that confer a selective advantage to the host bacterium. Studies in the last decade have made it amply clear that these two options do not exhaust the list of possible biological roles for R–M systems. Their presence in a cell may also have a bearing on other processes such as horizontal gene transfer and gene regulation. From genome sequencing and experimental data, we know that Bacillus anthracis encodes at least three methylation-dependent (typeIV) restriction endonucleases (RE), and an orphan DNA methyltransferase. In this article, we first present an outline of our current knowledge of R–M systems in B. anthracis. Based on available DNA sequence data, and on our current understanding of the functions of similar genes in other systems, we conclude with hypotheses on the possible roles of the three REs and the orphan DNA methyltransferase.
Highlights
The existence of restriction–modification (R–M) systems was initially inferred from the differential ability of bacteriophages to multiply on different strains of Escherichia coli (Arber and Dussoix, 1962; Arber et al, 1963)
Orphan DNA MTases encoded by phages, while protecting phage DNA from host R–M systems (Murphy et al, 2013), could conceivably contribute to gene regulation of host genes by methylation of regulatory sequences when integrated for the long term into the host genome as prophages
Examination of REBASE reveals that the methylation-dependent restriction enzymes (MDREs) in B. anthracis are only a subset of those observed in B. cereus and B. thuringiensis that possess MDREs in greater numbers and diversity
Summary
Reviewed by: Antoine Danchin, AMAbiotics SAS, France Shivangi Agarwal, Northwestern University, USA. Studies in the last decade have made it amply clear that these two options do not exhaust the list of possible biological roles for R–M systems. Their presence in a cell may have a bearing on other processes such as horizontal gene transfer and gene regulation. From genome sequencing and experimental data, we know that Bacillus anthracis encodes at least three methylation-dependent (typeIV) restriction endonucleases (RE), and an orphan DNA methyltransferase. Based on available DNA sequence data, and on our current understanding of the functions of similar genes in other systems, we conclude with hypotheses on the possible roles of the three REs and the orphan DNA methyltransferase
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