Question

Price et al. $[(1999).\text{ J. Bacteriol. }181:2358-2362]$ conducted a genetic study of the toxin transport protein (PA) of Bacillus anthracis, the bacterium that causes anthrax in humans. Within the 2294 -nucleotide gene in 26 strains they identified five point mutations- two missense and three synonyms-among different isolates. Necropsy samples from an anthrax outbreak in 1979 revealed a novel missense mutation and five unique nucleotide changes among ten victims. The authors concluded that these data indicate little or no horizontal transfer between different B. anthracis strains. (a) Which types of nucleotide changes (missense or synonyms) cause amino acid changes? (b) What is meant by "horizontal transfer"? (c) On what basis did the authors conclude that evidence of horizontal transfer is absent from their data?

Short answer

Answer: Missense mutations cause amino acid changes, resulting from a single nucleotide change that alters the codon and leads to a different amino acid being coded. Horizontal transfer, or horizontal gene transfer (HGT), refers to the transfer of genetic material between different species or organisms through a non-reproductive mechanism. In contrast, vertical transfer involves the transfer of genetic material from parent to offspring.

Step-by-step solution

Question (a): Types of nucleotide changes causing amino acid changes

Missense mutations are the type of nucleotide changes that cause amino acid changes. These mutations occur when a single nucleotide change results in a codon that codes for a different amino acid. As a result, the protein's primary structure changes, which may affect its function.

Question (b): Meaning of "horizontal transfer"

Horizontal transfer, also known as horizontal gene transfer (HGT), refers to the process by which genetic material is transferred between different species or organisms through a non-reproductive mechanism. This transfer can occur through various mechanisms such as transformation, conjugation or transduction and is different from the transfer of genetic material from parent to offspring (vertical transfer).

Question (c): Basis for the authors' conclusion on the absence of horizontal transfer

The authors based their conclusion on the absence of horizontal transfer in their data on the observation of a limited number of point mutations and unique nucleotide changes among the different B. anthracis strains. In the case of significant horizontal transfer, one would expect a higher degree of genetic diversity and a greater number of mutations, reflecting the exchange of genetic material between strains. However, the observed mutations were minimal, suggesting that horizontal transfer is little or non-existent among these strains of B. anthracis.

Key concepts

Missense Mutation

Missense mutations are a fascinating aspect of genetic mutations, defined by their ability to alter the amino acid sequence of proteins. A missense mutation occurs when a single nucleotide change results in a different codon, which then translates into a different amino acid. This small change at the DNA level can have significant implications for protein function, potentially altering a protein's structure or its biological activity. Think of it as changing one word in a sentence, which could alter the entire meaning of that sentence.

These mutations can be benign, deleterious, or sometimes even beneficial, depending on their impact on the protein's function. For instance, in Bacillus anthracis, the bacterium responsible for anthrax, missense mutations can be studied to understand their role in the toxin transport protein (PA). Understanding missense mutations helps researchers identify potential targets for medical interventions or important evolutionary adaptations.

Horizontal Gene Transfer

Horizontal gene transfer (HGT) is a critical concept in genetics and microbiology, highlighting the fascinating ways genetic material can move between organisms outside of traditional reproduction. Unlike vertical gene transfer, where genes are passed from parent to offspring, HGT facilitates genetic exchange between unrelated individuals or even different species.

This process can occur via several mechanisms including:

• Transformation: Uptake of free DNA from the environment by a cell.
• Conjugation: Direct transfer of DNA between two cells through a physical connection.
• Transduction: Transfer of DNA from one bacterium to another via a virus.

Horizontal gene transfer plays an essential role in the evolution of many organisms, allowing for rapid acquisition of new genes and traits, such as antibiotic resistance. However, the study by Price et al. found no significant evidence of HGT among Bacillus anthracis strains, indicating these bacteria evolve more via vertical transmission.

Bacillus anthracis Genetics

Bacillus anthracis is a bacterium with significant medical importance as the causative agent of anthrax. Understanding its genetic makeup is crucial for both disease treatment and prevention. The genetics of B. anthracis includes various fascinating aspects such as mutations and inheritance patterns that differ from other related species.

In the study conducted by Price et al., researchers focused on analyzing genetic variations of the toxin transport protein, PA. They discovered point mutations, including missense and synonymous types, in the B. anthracis genome. These findings revealed rare genetic changes within the studied strains, providing insights into the bacterium's evolutionary stability despite potential environmental pressures.

Understanding B. anthracis genetics aids in developing effective interventions and preventive measures against anthrax, showcasing the importance of genetic research in controlling infectious diseases.

Synonymous Mutation

Synonymous mutations, often seen as "silent" mutations, are fascinating because they do not change the resulting amino acid in the protein sequence. This occurs due to the redundancy of the genetic code; multiple codons can encode the same amino acid, which means a change in a single nucleotide might not affect the protein.

Despite this, synonymous mutations can still have biological impacts, such as affecting the rate of protein synthesis or mRNA stability. In the context of Bacillus anthracis research by Price et al., three synonymous mutations were found among B. anthracis strains, indicating genetic variability without altering the protein structure.

This type of mutation highlights the intricacy of genetic coding and its potential subtleties, underlining that even "silent" changes can influence an organism's fitness or adaptability in subtle ways.