DNA Methylation in Bacteria

How do bacteria respond to environmental challenges and signals so quickly and effectively? In addition to genetic modifications there are a series of non genetic, or epigenetic, modifications. Genetic modifications change the sequence of nucleotides that, for instance, comprise a protein-coding gene. In that case the resulting protein is modified to better handle the environmental challenge. Epigenetics, on the other hand, involves various other types of modifications. For instance, the three-dimensional structures of proteins may be dramatically altered, or tiny chemical signals—methyl groups—may be added to certain proteins or DNA sequences. As with genetic modifications, many of the epigenetic modifications are heritable, so the adjustments are passed on to later generations.

Consider the methylation of DNA. This occurs at certain target sites along the DNA sequence where specific short DNA sequences appear. These sequences are found by special proteins as they move along the DNA. The special proteins search for these sequences and add a methyl group to the adenine base that appears within the sequence. The protein binds to the DNA, twists the helix so the adenine base rotates into a precisely shaped pocket in the protein, and the protein then facilitates the transfer of the methyl group from a short donor molecule to the adenine.

The short donor molecule does more, however, than just supply a methyl group. It also actually helps to control the special protein. How does this work? Of course the short donor molecule binds to the pocket of the special protein so the methyl group is ready for transfer. But the donor molecule also binds to another site on the protein. This binding serves to alter the structure of the protein, enhancing its function. So the special protein is designed to do its job when it is charged with a donor molecule.

But not all of the DNA target sequences are methylated. This complex DNA methylation function doesn’t occur if the target sequence is protected by another protein that binds to the sequence. This protein binds to some of these DNA target sequences but not all. The result is a particular DNA methylation pattern which influences which genes are expressed, and therefore how the bacteria interacts with the environment.

This DNA methylation pattern is propagated to the daughter cells when the bacteria divides. When such division occurs the DNA must, of course, be replicated. The double helix is separated and new complementary strands are synthesized on each strand. At the DNA target sequences there is an adenine on both strands. If both adenines are methylated, then after replication the two newly formed DNA helices will each contain only a single methylated adenine—the original adenines are methylated but the new adenines that were added are not.

These hemimethylated sequences are rectified by other proteins, which methylate the lone, unmethylated, adenines. The result is that after cell division, the two new bacteria cells have inherited the full DNA methylation pattern established in the original cell. You can read more about this and other bacterial epigenetic mechanisms in this review paper.

This is one small chapter of the epigenetics story that helps to explain the incredible adaptation capabilities we observe in both single and multiple cell organisms. The idea that such capabilities evolved is, of course, not motivated by science. Evolutionists once claimed that adaptation was obvious proof of evolution, but in fact biological adapation is yet another massive problem for evolution.