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

Answer: Missense mutations cause amino acid changes in proteins, whereas synonyms do not. The difference between these two types of mutations is that a missense mutation is a change in a single nucleotide that results in a different amino acid being incorporated into a protein, altering its function, while a synonym mutation is a change in a single nucleotide that does not affect the amino acid produced due to the flexibility of the genetic code allowing some amino acids to be encoded by more than one codon.

Step-by-step solution

(a) Types of Nucleotide Changes

(A nucleotide change can be either missense or synonym. A missense mutation is a change in a single nucleotide that results in a different amino acid being incorporated into a protein, thereby altering its function. In contrast, a synonym mutation is a change in a single nucleotide that does not affect the amino acid produced, because some amino acids can be encoded by more than one codon. Therefore, missense mutations cause amino acid changes, while synonyms do not.)

(b) Horizontal Transfer Definition

(Horizontal transfer, also known as lateral transfer, refers to the transfer of genetic material between organisms in a method that is not directly inherited from a parent or ancestor. This can occur through mechanisms such as conjugation, transformation, or transduction. It results in the acquisition of new traits or genes in an organism, which can lead to increased genetic variety and the possibility for adaptation in populations.)

(c) Absence of Evidence for Horizontal Transfer

(The authors concluded that there was little or no evidence of horizontal transfer in their data based on the low amount of genetic variation observed among the different strains of Bacillus anthracis. They identified only five point mutations (two missense and three synonyms) in the 2294-nucleotide gene across 26 strains, and a novel missense mutation along with five unique nucleotide changes among ten victims of an anthrax outbreak. This low level of genetic diversity suggests that the strains are not frequently exchanging genetic material through horizontal transfer mechanisms, as higher genetic diversity would be expected if horizontal transfer were common.)