**2. Bacterial epigenetic mechanisms**

Modulation of chromosome organisation is one of the host defence mechanisms against bacterial attacks in eukaryotes. The host cell can often resist bacteria through these highly special and successful defence mechanisms. However, bacteria also have mechanisms that are developed against this system. Some bacteria may contain eukaryote-like proteins and eukaryotic histone translation proteins, which target the chromosomal machinery. In this way, they can activate appropriate enzymes in the host [16].

Several bacteria contain an N4-methylcytosine base whose function has not been fully characterised. There are studies indicating that these N4C modifications affect global gene expression in *Helicobacter pylori*, an example of carcinogenic bacteria [17].

Methylation in bacteria is different from eukaryotes in that it is seen in the fourth carbon of the cytosine as well as methylated adenine (N6-methyladenine) in addition to the fourth carbon of cytosine. DNA methylation occurs in bacteria by methyl binding to cytosine C-5 or N-4 and N6-adenine on the DNA methyltransferase enzyme side. N6-methyladenine is found only in bacteria and in less complex eukaryotes, and not in vertebrates [11]. Interestingly, bacteria also contain a restriction modification system that digests DNA methylase to provide protection against foreign DNA. These consist of the restriction enzyme systems called DcM, which recognises the 5-C cytosine, and Dam, which recognises methylated adenine. Of these, the Dam family is the most well-known protein group. The functional domain of Dam is a DNA MTase with an alpha molecule consisting of a polypeptide of 10 amino acids [18].

Similar to eukaryotes, bacteria also have rRNA methylation. The most important aspect of this methylation is that it creates targets for bacterial infections that can cause infection in humans. While promoter methylation is associated with negative expression, this may not always be the case for exon methylation. Still, sometimes exon methylation shows no effect on gene expression. Investigation on genetic mechanisms affecting cardiomyocyte differentiation includes some studies, which show that intragenic methylations create cellular memory through this mechanism, particularly in pluripotent cells [11].
