Question

Price et al. (1999. 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

(b) What is horizontal transfer and its significance in bacterial evolution? (c) What basis did the authors use to conclude that there was little or no horizontal transfer between different Bacillus anthracis strains?

Step-by-step solution

(a) Nucleotide changes and amino acid changes

There are two types of nucleotide changes: missense mutations and synonymous mutations. Missense mutations cause amino acid changes, as they result in different amino acids being coded for by the mutated codon. Synonymous mutations, on the other hand, do not cause amino acid changes, as they result in the same amino acid being coded for by the mutated codon, due to the degeneracy of the genetic code (multiple codons code for the same amino acid).

(b) Definition of horizontal transfer

Horizontal transfer, also known as horizontal gene transfer, is the transfer of genetic material between organisms by means other than reproduction (vertical transfer). This transfer can occur between members of the same species or between different species and can include mechanisms such as conjugation, transformation, and transduction. Horizontal transfer plays a significant role in the evolution of bacteria by allowing for the acquisition of new traits or the spread of antibiotic resistance genes among different bacterial strains.

(c) Basis for the authors' conclusion about the absence of horizontal transfer

The authors concluded that there was little or no horizontal transfer between different Bacillus anthracis strains based on their observation of a small number of point mutations (five in the 26 strains) and the relatively low diversity among the nucleotide sequences of the PA gene. If horizontal transfer was occurring frequently, they would expect to see a higher degree of genetic variation and a larger number of nucleotide changes among the different strains. In addition, the presence of a novel missense mutation and unique nucleotide changes among the necropsy samples further supported their conclusion that horizontal transfer was not a significant factor in this case.

Key concepts

Missense Mutations

Missense mutations are a type of genetic mutation where a single nucleotide change results in a different amino acid being incorporated into a protein. This occurs because the genetic code is altered, leading to a codon that specifies a different amino acid. The impact of a missense mutation can vary greatly:

• Sometimes it results in a functional protein that has slightly altered activity.
• Other times, it may severely affect the protein's function if the change occurs in a critical region of the protein structure.
• In some cases, missense mutations may even lead to diseases if the resultant protein malfunctions.

Understanding missense mutations is crucial in studying genetic diseases and developing corresponding therapies. These mutations emphasize the delicate balance and precision needed in protein synthesis, highlighting the importance of every codon in our genetic material.

Synonymous Mutations

Synonymous mutations, also known as silent mutations, involve changes to the DNA sequence that do not alter the amino acid sequence of a protein. This phenomenon is due to the redundancy of the genetic code. Multiple codons can code for the same amino acid, thus allowing changes in the DNA sequence without affecting the final protein. Here are some interesting aspects of synonymous mutations:

• While they do not change the protein sequence, they can still impact gene expression levels. This is because the speed of translation by ribosomes can vary depending on the codon used.
• These mutations can influence mRNA stability and splicing efficiency.
• Although synonymous mutations were once considered 'silent', research shows they can have subtle but significant effects on an organism.

Synonymous mutations demonstrate that even changes not directly altering proteins can play important roles in gene regulation.

Horizontal Gene Transfer

Horizontal gene transfer (HGT) refers to the sharing of genetic material between different organisms without the direct parent-offspring inheritance (vertical gene transfer). This process allows for genetic diversity and evolution among organisms, especially in bacteria. Key mechanisms of HGT include:

• Conjugation: Transfer of genetic material through direct contact between bacterial cells.
• Transformation: Uptake of free DNA from the environment by a bacterial cell.
• Transduction: Transfer of DNA from one bacterium to another via bacteriophages (viruses that infect bacteria).

HGT plays a critical role in bacterial evolution by:

• Allowing rapid acquisition of new advantages, like antibiotic resistance or virulence factors.
• Contributing to the genetic diversity within microbial populations.
• Facilitating adaptation to new ecological niches or environments.

Understanding HGT helps researchers grasp how genetic traits spread in populations, influencing fields like microbiology, evolution, and antibiotic development.