CHAPTER 10 Bacterial Genetics
Mutation is a heritable change in DNA sequence that can lead to a change in phenotype. By definition, a mutant differs from its parental strain in genotype, the nucleotide sequence of the genome. hisC (1,2,3,…) mutants of E. coli Auxotroph – nutritional mutant rewiring a growth factors, amino acids (e.g. His-). Prototroph – wild type
Selectable mutations are those that give the mutant a growth advantage under certain environmental conditions and are especially useful in genetic research. If selection is not possible, mutants must be identified by screening.
Although screening is always more tedious than selection, methods are available for screening large numbers of colonies in certain types of mutations. Penicillin selection – pen kills growing but not mutant (non growing) cells in minimal medium Nutritionally defective mutants can also be detected by the technique of replica plating (Figure 10.2).
Molecular Basis of Mutation Mutations, which can be either spontaneous or induced, arise because of changes in the base sequence of the nucleic acid of an organism's genome.
A point mutation, which results from a change in a single base pair, can lead to a single amino acid change in a polypeptide or to no change at all, depending on the particular codon involved (Figure 10.3).
In a nonsense mutation, the codon becomes a stop codon and an incomplete polypeptide is made. In a missense mutation, the sequence of amino acids in the ensuing polypeptide is changed, resulting in an inactive protein or one with reduced activity. Temperature-sensitive mutations/ conditionally lethal mutations
Deletions and insertions cause more dramatic changes in the DNA, including frameshift mutations, and often result in complete loss of gene function (Figure 10.4).
Mutation Rates Different types of mutations can occur at different frequencies. For a typical bacterium, mutation rates of 10–7 to 10–11 per base pair are generally seen.
Although RNA and DNA polymerases make errors at about the same rate, RNA genomes typically accumulate mutations at much higher frequencies than DNA genomes.
Mutagenesis Mutagens are chemical, physical, or biological agents that increase the mutation rate. Mutagens can alter DNA in many different ways, but such alterations are not mutations unless they can be inherited.
Table 10.2 gives an overview of some of the major chemical and physical mutagens and their modes of action.
There are several classes of chemical mutagens, one being the nucleotide base analogs (Figure 10.5).
Some DNA damage can lead to cell death if not repaired. A complex cellular mechanism called the SOS regulatory system is activated as a result of some types of DNA damage and initiates a number of DNA repair processes, both error-prone and high-fidelity (Figure 10.7).
The Ames test employs a sensitive bacterial assay system for detecting chemical mutagens in the environment.
Genetic Recombination Homologous recombination arises when closely related DNA sequences from two distinct genetic elements are combined in a single element (Figure 10.9).
Recombination is an important evolutionary process, and cells have specific mechanisms for ensuring that recombination takes place.
Mechanisms of recombination that occur in prokaryotes involve DNA transfer during the processes of transformation, transduction, and conjugation (Figure 10.11).
The discovery of transformation was one of the seminal events in biology because it led to experiments demonstrating that DNA is the genetic material (Figure 10.13).
Certain prokaryotes exhibit competence, a state in which cells are able to take up free DNA released by other bacteria.
Incorporation of donor DNA into a recipient cell requires the activity of single-stranded binding protein, RecA protein, and several other enzymes. Only competent cells are transformable (Figure 10.14).
Transduction Transduction involves the transfer of host genes from one bacterium to another by bacterial viruses.
In generalized transduction (Figure 10.15), defective virus particles incorporate fragments of the cell's chromosomal DNA randomly, but the efficiency is low.
In specialized transduction (Figure 10.16), the DNA of a temperate virus excises incorrectly and takes adjacent host genes along with it; transducing efficiency in this case may be very high.
Plasmids: General Principles Plasmids are small circular or linear DNA molecules that carry any of a variety of unessential genes. Although a cell can contain more than one plasmid, they cannot be closely related genetically.
Figure 10.18 shows a genetic map of the F (fertility) plasmid, a very well characterized plasmid of Escherichia coli.
Lateral transfer in the process of conjugation can transfer plasmids (Figure 10.19).
The genetic information that plasmids carry is not essential for cell function under all conditions but may confer a selective growth advantage under certain conditions.
Examples include antibiotic resistance (Figure 10.20), enzymes for degradation of unusual organic compounds, and special metabolic pathways. Virulence factors of many pathogenic bacteria are often plasmid-encoded.