Question

In E coli, a methyltransferase encyme encoded by the dam gene recognizes the sequence \(5^{\prime}-\mathrm{GATC}-3\) and attaches a methyl group to the nitrogen at position 6 of adenine. E. coli strains that have the dam gene deleted are known to have a higher spontaneous mutation rate than normal strains. Explain why.

Short answer

The dam gene in E. coli encodes a methyltransferase enzyme that carries out DNA methylation, a process playing a key role in gene regulation and DNA stability. Absence of the dam gene means absence of the enzyme and thus DNA methylation doesn't occur. This leaves the bacterial DNA more susceptible to mutations, thereby increasing the spontaneous mutation rate.

Step-by-step solution

Understanding the role of the dam gene and its encoded methyltransferase enzyme

The dam gene in E. coli encodes a special enzyme called a methyltransferase. This enzyme recognizes the DNA sequence \(5^{\prime}-\mathrm{GATC}-3\) and adds a methyl group to the nitrogen atom at the 6th position of adenine. This process is known as DNA methylation.

Understanding the function of DNA methylation

DNA methylation serves as a means of regulating gene expression. It also helps stabilize the DNA molecule, preventing it from spontaneous mutations by reducing the chance of transposon or similar mobile genetic element movement. It serves as a defensive mechanism against the integration of foreign DNA, and also plays a role in DNA mismatch repair (a system for recognizing and repairing erroneous insertion, deletion, and mis-incorporation of bases that can arise during DNA replication and recombination).

Linking the absence of the dam gene to a higher spontaneous mutation rate

In E. coli strains where the dam gene is deleted, the methyltransferase enzyme is not produced, so DNA methylation cannot occur. This means that the effect of stabilization and protection offered by methylation is lost. Consequently, the DNA becomes more susceptible to mutations by increasing the chance of transposon movement, improper integration of foreign DNA, and errors in DNA replication and recombination. This leads to a higher spontaneous mutation rate in these strains compared to normal strains that have intact dam genes and thus effective DNA methylation machinery.

Key concepts

dam Gene Function

In the world of bacteria like Escherichia coli (E. coli), the dam gene plays a crucial role in maintaining the integrity of the organism's genetic information. The term 'dam' stands for DNA adenine methyltransferase. The primary function of the dam gene is to encode the creation of an enzyme known as a methyltransferase. This enzyme's job is to patrol the E. coli DNA and add a methyl group, which is a small chemical tag, to the adenine base at a specific DNA sequence: 5' - GATC - 3'. The addition of this methyl group is a form of DNA methylation.

Why is this important? DNA methylation serves multiple purposes: it is a genetic bookmark of sorts, indicating which genes should be active and which should be silent. Moreover, the methylation at the GATC sequence plays a protective role by stabilizing the DNA molecule. This stability is essential for preventing spontaneous mutations — unpredictable changes to the DNA sequence that can have a variety of consequences. Additionally, methylation signals are used during the process of DNA mismatch repair, a cellular mechanism that corrects copy errors introduced during DNA replication. In the absence of a functioning dam gene, these critical processes are disrupted, leading to increased vulnerability to mutations and potential harm to the organism.

Methyltransferase Enzyme

Methyltransferase is an enzyme that acts as a meticulous molecular scribe, carefully modifying the genetic material of E. coli. Specifically, it's in charge of adding a methyl group — CH3 — onto the adenine base at the sixth nitrogen position within the GATC sequence. Imagine the enzyme as a tiny editor, making sure each 'word' in the DNA sequence contains the right 'punctuation' by methylating specific sites.

This biochemical punctuation does more than just decorate the DNA; it's crucial for distinguishing between the original DNA strand and the newly synthesized strand during DNA replication. As new DNA strands are formed, they initially lack the methyl groups, making them 'unmarked'. The methylation pattern, or the lack of one on new strands, provides cues for other cellular machinery engaged in DNA repair and replication fidelity. Methylation instructs repair enzymes to focus on the newer, unmethylated strand when fixing replication errors. When the enzyme responsible for this task is missing or not functioning correctly, E. coli's genetic blueprint can become riddled with errors more rapidly, leading to that higher spontaneous mutation rate observed in strains lacking a functional dam gene.

Spontaneous Mutation Rate

Mutation is like a typo in the genetic code of an organism, and a spontaneous mutation is one that happens without an external cause, like radiation or chemicals. In E. coli, these natural genetic typos can occur at a particular pace, which we call the spontaneous mutation rate. It essentially tells us how often random mutations arise within the E. coli genome without outside influence.

The spontaneous mutation rate is of profound importance because it impacts the organism's ability to adapt and evolve. While some mutations can be beneficial, providing genetic variation that can be advantageous in changing environments, most are neutral or detrimental. Too many harmful mutations can lead to genetic disorders or reduced fitness. In a typical dam-positive E. coli strain, the methyltransferase enzyme works as a safeguard, preserving the genetic integrity by facilitating the repair of mispaired bases that can lead to mutations. When E. coli lacks this safeguard due to a deleted dam gene, the guardrails are gone. This gap in genomic security means the spontaneous mutation rate goes up, as the usual patrol and correction mechanisms are not in place, allowing more mistakes to slip through the genetic net and become permanent fixtures in the organism's DNA.