Abstract
The highly similar prokaryotic DNA (cytosine-5) methyltransferases (C5-MTases) M.MpeI and M.SssI share the specificity of eukaryotic C5-MTases (5’-CG), and can be useful research tools in the study of eukaryotic DNA methylation and epigenetic regulation. In an effort to improve the stability and solubility of complementing fragments of the two MTases, genes encoding circularly permuted (CP) variants of M.MpeI and M.SssI were created, and cloned in a plasmid vector downstream of an arabinose-inducible promoter. MTase activity of the CP variants was tested by digestion of the plasmids with methylation-sensitive restriction enzymes. Eleven of the fourteen M.MpeI permutants and six of the seven M.SssI permutants had detectable MTase activity as indicated by the full or partial protection of the plasmid carrying the cpMTase gene. Permutants cp62M.MpeI and cp58M.SssI, in which the new N-termini are located between conserved motifs II and III, had by far the highest activity. The activity of cp62M.MpeI was comparable to the activity of wild-type M.MpeI. Based on the location of the split sites, the permutants possessing MTase activity can be classified in ten types. Although most permutation sites were designed to fall outside of conserved motifs, and the MTase activity of the permutants measured in cell extracts was in most cases substantially lower than that of the wild-type enzyme, the high proportion of circular permutation topologies compatible with MTase activity is remarkable, and is a new evidence for the structural plasticity of C5-MTases. A computer search of the REBASE database identified putative C5-MTases with CP arrangement. Interestingly, all natural circularly permuted C5-MTases appear to represent only one of the ten types of permutation topology created in this work.
Highlights
DNA methylation plays important roles in several biological phenomena such as restrictionmodification in prokaryotes, genomic imprinting, X-chromosome inactivation and silencing of selfish genetic elements in eukaryotes
Permutation sites were designed with the web-based tool Circular Permutation Site Predictor (CPred) [39]
There were a few known exceptions, enzymes in which the order of conserved motifs was circularly permuted relative to the canonical order: M.BssHII [11], [12], M.Alw26I, M2.Eco31IC, M.Esp3I [13] and M2.BsaI [14] and Zhu and Xu, cited in REBASE [15])
Summary
DNA methylation plays important roles in several biological phenomena such as restrictionmodification in prokaryotes, genomic imprinting, X-chromosome inactivation and silencing of selfish genetic elements in eukaryotes. Biological DNA methylation is catalyzed by DNA methyltransferases (DNA MTase), which transfer a methyl group from the universal methyl. Depending on the methylated base, DNA MTases can be classified in three groups N6-adenine and N4-cytosine MTases transfer the methyl group onto the exocyclic amino group of the respective base, whereas DNA (cytosine-5) methyltransferases (C5-MTases) add the methyl group to carbon 5 of the pyrimidine ring [3]. In which all three types DNA methylation are ubiquitous, eukaryotes typically contain C5-methylcytosine [1, 2]. In higher eukaryotes C5-methylcytosines are important epigenetic marks [4], which occur predominantly in CG dinucleotides
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