Final answer:
The methylation of bacterial DNA primarily occurs in CpG islands within the promoter regions of genes, which influences gene expression and can result in gene silencing. It also serves as a defense mechanism to protect bacterial DNA from enzymatic cleavage.
Step-by-step explanation:
The bacterial genome undergoes a modification called DNA methylation. This process occurs primarily within specific areas known as CpG islands. These are regions that have a high frequency of cytosine (C) and guanine (G) dinucleotide DNA pairs, which are frequently found in the promoter regions of genes. Methylation involves adding a methyl group to the cytosine base of the CG pair. This alteration impacts how the DNA interacts with various proteins—including histone proteins that regulate the accessibility of DNA for transcription—resulting in gene silencing or other regulatory effects.
Furthermore, in bacteria, this methylation serves as a defense mechanism. Bacterial cells contain DNA methyltransferases that methylate their DNA at specific short, palindromic sequences. This modification helps protect the host DNA from cleavage by restriction enzymes, which instead target unmethylated foreign DNA, a process central to bacterial immunity against viral infections. In E. coli, which has a supercoiled genome of 4.6 million base pairs fitted within the bacterial cell by supercoiling aided by DNA gyrase, methylation plays a crucial role in genomic stability and protection.
Overall, DNA methylation and supercoiling are critical for managing and protecting bacterial genomes, ensuring proper gene regulation, and defending against foreign DNA intrusion.