The host specificity system in Escherichia coli SK

Means, mechanisms and consequences of adenine methylation in DNA

N⁶-methyl-2'-deoxyadenosine (6mA or m⁶dA) has been reported in the DNA of prokaryotes and eukaryotes ranging from unicellular protozoa and algae to multicellular plants and mammals. It has been proposed to modulate DNA structure and transcription, transmit information across generations and have a role in disease, among other functions. However, its existence in more recently evolved eukaryotes remains a topic of debate. Recent technological advancements have facilitated the identification and quantification of 6mA even when the modification is exceptionally rare, but each approach has limitations. Critical assessment of existing data, rigorous design of future studies and further development of methods will be required to confirm the presence and biological functions of 6mA in multicellular eukaryotes.

N6-Methyladenine: A Conserved and Dynamic DNA Mark

Chromatin, consisting of deoxyribonucleic acid (DNA) wrapped around histone proteins, facilitates DNA compaction and allows identical DNA codes to confer many different cellular phenotypes. This biological versatility is accomplished in large part by posttranslational modifications to histones and chemical modifications to DNA. These modifications direct the cellular machinery to expand or compact specific chromatin regions and mark regions of the DNA as important for cellular functions. While each of the four bases that make up DNA can be modified (Iyer et al. 2011), this chapter will focus on methylation of the sixth position on adenines (6mA), as this modification has been poorly characterized in recently evolved eukaryotes, but shows promise as a new conserved layer of epigenetic regulation. 6mA was previously thought to be restricted to unicellular organisms, but recent work has revealed its presence in metazoa. Here, we will briefly describe the history of 6mA, examine its evolutionary conservation, and evaluate the current methods for detecting 6mA. We will discuss the enzymes that bind and regulate this mark and finally examine known and potential functions of 6mA in eukaryotes.

On heterogeneity of DNA methylases from Escherichia coli SK cells

The presence of E. coli SK cells of five different DNA-methylases differing in specificity to the methylated sequence is documented has been proven. Two enzymes methylate cytosine with the formation of 5'-methylcytosine and three enzymes methylate adenine with formation of 6'-methylaminopurine. A method for simultaneous isolation of the five individual enzymes including gel filtration on Biogel A-0.5 M is proposed. The direct evidence has been presented showing that the additional methylation test in our method modification actually can discriminate between enzymes differing in sensitive sites.

Determination of the recognition sites of cytosine DNA-methylases from Escherichia coli SK

Two different cytosine DNA-methylases, NI and GII, are present in Escherichia coli SK. The GII methylase recognizes the five-member symmetric sequence: 5'...NpCpCpApGpGpN...3'. This sequence is identical with the recognition site of the hsp II type determined by RII plasmid but, in contrast to RII methylase, the GII enzyme methylates cytosine located on the 5' side of the site. By analogy with the isoshizomery of the restricting endonucleases, RII and GII DNA methylaeses may be called isomethymers which recognize the same site but methylate different bases. Since the phage of the SK and hsp II phenotypes is effectively restricted in respective cells it may be assumed that the isomethymeric modification does not provide any protection against the corresponding restrictases. NI methylase recognizes the five-member symmetric site which represents an inverted sequence of the GII site: 5'...NpGpGpApCpCpN...3'. In this case cytosine at the 3'-end of the recognition site is methylated.