Besides of containing methyl- cytosines, it is known that DNA also includes a sixth base, N6- methyl adenine (m6A), which is essential for the viability of bacteria, but also occurs in archaea and eukaryotes. In bacteria, adenine methylation is associated with the protection of the DNA from endonucleases, via the restriction- modification system, and the enzymes involved (adenine methyltransferases), have been also found to be encoded by phage genomes. On the other hand, adenine methylation marks are also used to identify parental and nascent DNA strands during chromosome replication and DNA mismatch repair.
In eukaryotes, m6A and its endonucleases and methyl- transferases are coded in some protists like Chlorella, which might possibly protect this unicellular organism from infection. In ciliate protozoa, m6A is only found in macronuclear DNA only, and occurring with a non- random distribution. However, the assumption that m6A is not present in higher eukaryotes, which comes from experiments made in the 1970s, is somewhat evolutively surprising if we consider all its functions in bacteria, not to mention the high mutability of m5C to thymine (which is supposed to be a defense against genomic parasites such as transposons). Hence, the large number of transposons in eukaryotic genomes could have masked the small amounts of m6A, and thus this base has not been as throughly investigated as the other epigenetic mark. Indeed, there are reports of m6A in higher eukaryotes, particularly plant and mosquito DNA.
[To date of this paper], in mammals there is only indirect evidence of m6A in rodents (from sensitive restrictases assays), but the definitive physical detection is still needed. It seems that in these studied mammalian genes (mouse Myo-D1 and rat steroid reductase), m6A could be modulating gene expression, since it was showed that it interferes with protein – DNA binding. This could be a matter of concern considering that plasmids for transient expression are propagated in bacterial cells before introduced to animal cell cultures.
This controversy of the presence or not of m6A in higher eukaryotes is reminiscent of the one that arose regarding the presence of 5mC in Drosophila, which was just demonstrated in this decade. Thus, more sensitive approaches are required to clarify its presence, for example using HPLC coupled to tandem mass spectrometry, and technological limitations could be indirectly solved if the corresponding methyl- transferase is found. Authors proposed that m6A could function as a defense mechanism, as it is for bacterial chromosomes, and also to mark the “immortal” DNA strand of adult stem cells, that divides asymmetrically.
In eukaryotes, m6A and its endonucleases and methyl- transferases are coded in some protists like Chlorella, which might possibly protect this unicellular organism from infection. In ciliate protozoa, m6A is only found in macronuclear DNA only, and occurring with a non- random distribution. However, the assumption that m6A is not present in higher eukaryotes, which comes from experiments made in the 1970s, is somewhat evolutively surprising if we consider all its functions in bacteria, not to mention the high mutability of m5C to thymine (which is supposed to be a defense against genomic parasites such as transposons). Hence, the large number of transposons in eukaryotic genomes could have masked the small amounts of m6A, and thus this base has not been as throughly investigated as the other epigenetic mark. Indeed, there are reports of m6A in higher eukaryotes, particularly plant and mosquito DNA.
[To date of this paper], in mammals there is only indirect evidence of m6A in rodents (from sensitive restrictases assays), but the definitive physical detection is still needed. It seems that in these studied mammalian genes (mouse Myo-D1 and rat steroid reductase), m6A could be modulating gene expression, since it was showed that it interferes with protein – DNA binding. This could be a matter of concern considering that plasmids for transient expression are propagated in bacterial cells before introduced to animal cell cultures.
This controversy of the presence or not of m6A in higher eukaryotes is reminiscent of the one that arose regarding the presence of 5mC in Drosophila, which was just demonstrated in this decade. Thus, more sensitive approaches are required to clarify its presence, for example using HPLC coupled to tandem mass spectrometry, and technological limitations could be indirectly solved if the corresponding methyl- transferase is found. Authors proposed that m6A could function as a defense mechanism, as it is for bacterial chromosomes, and also to mark the “immortal” DNA strand of adult stem cells, that divides asymmetrically.
For several decades now, the importance of epigenetic mechanisms involving DNA methylation in mammals has resulted in m5C being considered as the fifth base of DNA. However, it seems likely that the relative high abundance of m5C in mammalian DNA has focused attention on the role of m5C to the detriment of investigations on m6A. The fundamental role played by m6A in bacteria raises fascinating questions on its phyletic distribution and on its possible functions in eukaryotes. Hence, determining unambiguously whether our genome is made of five or six bases, and elucidating the biological functions played by m6A in eukaryotes is now a matter of crucial concern for both basic research and drug development.Ratel, D. et al. 2006. BioEssays 28: 309-315
epigenetics
