Microbial Genetics: Mutations and Recombination

Molecular Basis and Evolutionary Role of Mutations

At the molecular level, a mutation is defined as a specific alteration in the nitrogen base sequence of DNA. This change in the genotype often leads to a phenotypic change, which is the observable manifestation of the genetic alteration. Such changes can manifest as altered gene expression, including the appearance or disappearance of anatomical or physiological traits. For instance, a pigmented bacterium might lose its capacity to synthesize pigment, or a specific strain of a malarial parasite might develop resistance to a pharmaceutical drug. Genetic change, facilitated by mutations, serves as the fundamental driving force of evolution.

Characterization of Wild Type and Mutant Strains

A microorganism exhibiting its natural, nonmutated characteristics is referred to as the wild type or wild strain. Conversely, if an organism harbors a mutation, it is classified as a mutant strain. These mutant strains often display variances across a wide range of characteristics, including morphology, nutritional requirements, genetic control mechanisms, resistance to chemicals, temperature preferences, and nearly any type of enzymatic function. Mutant strains are essential tools in microbiology for tracking genetic events, unraveling genetic information, and identifying specific genetic pathways. Recent real-world examples of wild types and their corresponding mutant strains include the COVID-19 virus. The original strain is the wild type, while variants such as Alpha, Delta, Omicron, and the SA variant are categorized as mutant strains.

Functional Categories of Mutations

Point mutations are genetic alterations that affect only a single base within a gene, involving the addition, deletion, or substitution of single bases. If such a permanent alteration is faithfully copied into mRNA and subsequently translated, it can drastically change the structure of the resulting protein. Within this category, mutations are further classified by their biological impact. Lethal mutations have a harmful effect on the cell, typically leading to severe dysfunction or cell death. Neutral mutations are those that produce neither adverse nor beneficial changes to the organism's survival.

Specific Types of Substitution and Frameshift Mutations

Substitution mutations include missense, nonsense, and silent mutations. A missense mutation involves a change in the genetic code that results in the placement of a different amino acid in the protein sequence. This can lead to a faulty or nonfunctional protein, a protein that functions differently, or potentially cause no significant alteration. A nonsense mutation changes a normal codon into a stop codon, which halts protein production prematurely and almost always results in a nonfunctional protein. A silent mutation alters a DNA base but does not change the resulting amino acid due to the redundancy of the genetic code; for example, the codons ACU, ACC, ACG, and ACA all code for threonine, meaning a mutation at the third position would have no effect on the protein structure.

Frameshift mutations occur when the reading frame of the mRNA is altered due to the insertion or deletion of one or more bases in the DNA strand. Because the genetic code is read in triplets, a frameshift causes every amino acid following the site of the mutation to be different from the original code. This almost inevitably results in a nonfunctional protein. An illustrative linguistic analogy for these mutations is the sentence: "THE BIG BAD DOG ATE THE FAT RED CAT." A missense mutation might change it to "THE BIG BAD DOG ATE THE FIT RED CAT," while a nonsense mutation might truncate it to "THE BIG BAD (stop)." A frameshift insertion would shift every letter: "THE BIG BAB DDO GAT ETH EFA TRE DCA T."

Origins and Causes of Mutations: Spontaneous vs. Induced

Mutations are classified by their origin as either spontaneous or induced. Spontaneous mutations are random changes in DNA arising from errors during replication that occur without a known external cause. The frequency of these events has been measured across various organisms. Induced mutations result from exposure to known mutagens, which are physical or chemical agents that damage DNA. Mutagens are often used in controlled laboratory settings to induce mutations for scientific study.

Chemical mutagens include Nitrous acid and bisulfites, which remove amino groups from bases; Ethidium bromide, which inserts between paired bases; and Acridine dyes, which cause frameshifts. Nitrogen base analogs compete with natural bases for sites on replicating DNA. Radiation also acts as a mutagen: Ionizing radiation (gamma rays and X-rays) forms free radicals that cause single or double-strand breaks in DNA, while Ultraviolet (UV) radiation causes cross-links between adjacent pyrimidines.

Methods for Detecting and Isolating Mutant Bacteria

Detecting mutant bacteria often involves using solid media containing differential or selective agents, such as specific metabolic substrates or antibiotics. For example, when wild-type E. coli is grown on MacConkey agar (a differential medium), most colonies will be positive for lactose fermentation. However, a small number of mutant colonies that have lost the genes for using lactose will appear negative and can be isolated. Antibiotics are also used as selective agents to allow only resistant mutants to grow.

Another significant method is the replica plating technique, developed by Joshua Lederberg. This technique identifies mutants based on nutritional deficiencies. A culture is exposed to a mutagen and plated on a complete medium containing all necessary nutrients. The colonies are then transferred using a sterile "pickup" or replica carrier (often velvet) onto two separate plates: one complete medium and one incomplete medium lacking a specific nutrient. By comparing the two plates, researchers can identify mutant colonies that grow on the complete medium but fail to grow on the incomplete medium, indicating they have lost the ability to synthesize that specific nutrient.

DNA Repair Mechanisms

Cells possess sophisticated mechanisms to repair DNA damage, as mutations can be life-threatening. Photoactivation, or light repair, is used to restore DNA damaged by UV radiation. This process requires visible light and the enzyme DNA photolyase, which attaches to abnormal pyrimidine bonds and restores the original structure. However, this is only effective for a small number of UV-induced mutations. Excision repair is a broader mechanism where a series of enzymes recognize, excise, and replace incorrect bases. First, enzymes break the bonds in the sugar-phosphate backbone; then, the defective bases are removed. DNA polymerase I fills the resulting gap with the correct bases, and ligase seals the strand. This system can also fix mismatched pairs missed during proofreading, such as $C$ mistakenly paired with $A$, or $G$ with $T$.

The Ames Test for Carcinogenicity

The Ames test is a rapid screening method used to detect chemicals with carcinogenic (cancer-causing) potential. It uses a mutant strain of Salmonella enterica that lacks the ability to synthesize the amino acid histidine ($his(-)$). The premise is that any chemical capable of mutating bacterial DNA is likely to mutate human DNA as well. The test measures "back-mutation" or reversion; if a test agent increases the rate at which the bacteria mutate back to the wild type ($his(+)$), it is considered a mutagen. Because many chemicals only become mutagenic after processing by the liver, an extract of mammalian liver enzymes is often added to the test medium to simulate human metabolism.

Genetic Recombination and Horizontal Gene Transfer

While bacteria do not undergo sexual reproduction, they utilize genetic recombination to share and recombine genomic segments. Recombination involves a donor bacterium transferring DNA to a recipient, resulting in a new strain distinct from both parents. These events are generally beneficial, providing genes for drug resistance, metabolic capabilities, or increased virulence. There are three primary modes of transmission: Conjugation, Transformation, and Transduction.

Conjugation involves direct contact between two cells, typically via a pilus. In gram-negative bacteria, an $F$ factor (fertility plasmid) directs the synthesis of a sex pilus that draws the recipient cell close to the donor. A mating bridge forms, and the plasmid is transferred. High-frequency recombination ($Hfr$) occurs when the $F$ factor integrates into the bacterial chromosome, allowing chromosomal genes to be transferred. This process is vital for spreading Resistance ($R$) factors, which carry genes for antibiotic and heavy metal resistance.

Transformation is the capture of "naked" DNA from the environment by a competent cell. This was famously demonstrated by Frederick Griffith in the 1920s using Streptococcus pneumoniae. He showed that nonvirulent Rough ($R$) strains could be transformed into virulent Smooth ($S$) strains after taking up DNA from heat-killed $S$ cells. Transduction is DNA transfer mediated by a bacteriophage (virus). In generalized transduction, random segments of host DNA are accidentally packaged into a phage. In specialized transduction, a prophage excises itself from the host chromosome and takes specific adjacent host genes with it, transferring them to the next host cell.

Transposons: "Jumping Genes"

Transposons are segments of DNA capable of shifting from one location in the genome to another. Originally discovered by Barbara McClintock in corn plants, these "jumping genes" are now known to exist in all prokaryotic and eukaryotic cells, as well as viruses. They comprise approximately $45\%$ of the human genome. Transposons can move between chromosomal sites, or between chromosomes and plasmids. They contain genes for enzymes required for excision and reintegration, flanked by sequences called inverted repeats. Some transposons replicate before moving, increasing their copy number within the genome, while others move without replication.