3.1: Horizontal gene transfer in bacteria (2023)

Learning goals

After completing this section, you should be able to accomplish the following objectives.

1. Compare and contrast mutation and horizontal gene transfer as methods by which bacteria can respond to selective pressure and adapt to new environments.
2. Define horizontal gene transfer and name the most common form of horizontal gene transfer in bacteria.
3. Briefly describe the transformation mechanisms in bacteria.
4. Briefly describe the following mechanisms of horizontal gene transfer in bacteria:
   1. generalized transduction
   2. specialized transduction
5. Briefly describe the following mechanisms of horizontal gene transfer in bacteria:
   1. Transfer of conjugative plasmids, conjugative transposons and mobilizing plasmids in gram-negative bacteria
   2. F⁺conjugation
   3. hfr conjugation
6. Describe R plasmids and the importance of R plasmids for medical microbiology.

Transformation

Transformation is a form of genetic recombination in which a piece of DNA from a dead and degraded bacterium enters a competent recipient bacterium and is replaced by a piece of DNA from the recipient. Transformation generally only involves homologous recombination, the recombination of homologous regions of DNA that have nearly identical nucleotide sequences. Usually this includes similar strains of bacteria or strains of the same species of bacteria.

Some bacteria, e.gNeisseria gonorrhoeae, Neisseria meningitidis, Haemophilus influenzae, Legionella pneumophila, Streptococcus pneumoniae, GHelicobacter pyloriThey tend to be naturally capable and transformable. Competent bacteria can bind much more DNA than incompetent bacteria. Some of these lines also undergo autolysis which then yields DNA for homologous recombination. In addition, some competent bacteria kill incompetent cells to release DNA for transformation.

During transformation, DNA fragments (usually about 10 genes long) are released from the dead, degraded bacterium and bind to DNA-binding proteins on the surface of the competent living recipient bacterium. Depending on the bacteria, both DNA strands enter the receptor, or the nuclease breaks off one strand of the fragment and the remaining DNA strand enters the receptor. This fragment of donor DNA is then exchanged for a piece of recipient DNA by the RecA protein and other molecules and involves breaking and rejoining the paired DNA segments, as shown in (Figure \(\PageIndex{1 } \)). The transformation is summarized in the figure \(\PageIndex{2}\).

Image \(\PageIndex{2}\): Transformation: Step 1: The donor bacteria dies and decomposes. Step 2:Pieces of DNA, usually about 10 genes long, from the killed donor bacteria are added to thetransformasomiin the cell wall of living and competent recipient bacteria. Step 3:In this example, the nuclease cleaves one strand of the donor fragment and the remaining DNA strand enters the recipient. Competition-specific single-stranded DNA-binding proteins bind to the donor DNA strand to prevent its degradation in the cytoplasm. Step 4: RecA proteins promote genetic exchange between donor and recipient DNA fragments (see figure \(\PageIndex{1}\) for RecA protein functions). It involves breaking and rejoining paired segments of DNA. Step 5:The transformation is complete.

transduction

Transduction is the transfer of a DNA fragment from one bacterium to another by means of a bacteriophage. There are two forms of transduction:generalized transductionGspecialized transduction.

During replicationlytische bacteriofagenGtemperate bacteriophagesSometimes the phage capsid accidentally gathers around a small piece of bacterial DNA. When this bacteriophage, called a transducer particle, infects another bacterium, it injects a fragment of the donor bacterium's DNA that it carries into the recipient, where it can later be exchanged for some of the recipient's DNA by homologous recombination. Generalized transduction is summarized in figure \(\PageIndex{3}\).

• Paso 1: Bacteriophage is adsorbed on a sensitive bacteria.
• Step 2: The genome of the bacteriophage penetrates the bacterium. The genome directs the metabolic machinery of the bacterium to produce enzymes and bacteriophage components. Bacteriophage-encoded enzymes will also break down the bacterial chromosome.
• Paso 3:Occasionally, the bacteriophage capsid accidentally assembles around a chromosome fragment of the donor bacterium or around a plasmid instead of around the phage genome.
• phase 4: Bacteriophages are released when bacteria are lysed. Note that the bacteriophage carries a DNA fragment of the donor bacterium, not the genome of the bacteriophage.
• Paso 5:Bacteriophage carrying the DNA of the donor bacterium is adsorbed to the recipient bacterium.
• Paso 6:The bacteriophage transfers the DNA of the donor bacterium it carries into the recipient bacterium.
• Paso 7:Homologous recombination takes place and the donor bacteria's DNA is exchanged for part of the recipient's DNA. (Figure \(\PageIndex{1}\) shows the functions of RecA proteins involved in homologous recombination).

Generalized transduction occurs in a variety of bacteria, includingStaphylococcus, Escherichia, Salmonella, GPseudomonas.

Plasmids, such as the penicillinase plasmidStaphylococcus aureus, can also be transferred from one bacterium to another by generalized transduction.

Specialized Transduction: This may occasionally occur during the lysogenic life cycle of a temperate bacteriophage. In spontaneous induction, a small piece of bacterial DNA can sometimes be exchanged for part of the bacteriophage genome, which remains in the bacterial nucleoid. This part of the bacterial DNA is replicated as part of the bacteriophage genome and is located on the capsid of each phage. Bacteriophages are released, adsorbed to the recipient bacteria and inject the donor bacteria DNA/phage DNA complex into the recipient bacteria, where it is inserted into the bacterial chromosome (Figure \(\PageIndex{4}\)).

Conjugation

Genetic recombination in which DNA is transferred from a living donor bacterium to a living recipient bacterium through cell-to-cell contact. Gram-negative bacteria are usually aconjugationOsix hairs.

Conjugation is encoded by plasmids or transposons. It is a donor bacteria that contains a conjugative plasmid and a recipient cell that does not. A conjugated plasmid is self-transferable, as it has all the genes necessary to transfer that plasmid to another bacterium by conjugation. conjugation genes known asBetweenGenes enable the bacteria to form a mating pair with another organismoTThe sequences (origin of the transfer) determine where on the plasmid DNA transfer begins and serve as the replication initiation site where DNA replication enzymes will cut the DNA to initiate DNA replication and transfer. In addition, mobilizable plasmids are missingBetweengenes for autotransmissibility, but possessoTDNA transfer initiation sequences can also be transferred conjugally if the bacterium containing them also possesses a conjugative plasmid. HeBetweenThe genes on the conjugative plasmid enable pairingoTof a plasmid that can be mobilized allows DNA to move across the conjugation bridge (image \(\PageIndex{5}\)).

Transposons ("jumping genes") are small pieces of DNA that code for enzymes that allow the transposon to move from one DNA site to another, either on the same DNA molecule or on a different molecule. Transposons can be found as part of the bacterial chromosome (conjugative transposons) or on plasmids and are generally between one and twelve genes in length. A transposon contains several genes, such as genes that code for antibiotic resistance or other traits, flanked at both ends by insertion sequences that code for an enzyme called a transpoase. Transpoase is an enzyme that catalyzes the cutting and reclosing of DNA during transposition.

Conjugative transposons, as well as conjugative plasmids, carry genes that enable the formation of pairs of conjugative pairs. Therefore, conjugative transposons also allow the transfer of mobilizing plasmids and non-conjugative transposons to the recipient bacterium during conjugation.

Many conjugative plasmids and conjugative transposons have rather promiscuous transfer systems that allow them to transfer DNA not only to similar species, but also to unrelated species. The ability of bacteria to adapt to new environments as part of bacterial evolution usually results from the acquisition of large DNA sequences from another bacteria through conjugation.

Unpleasant. General mechanism of transfer of conjugated plasmids by conjugation in gram-negative bacteria

In Gram-negative bacteria, the first step in conjugation involves the conjugation pilus (gena pilus or F-pilus) on the donor bacterium binding to a recipient bacterium that lacks the conjugation pilus. Normally, the conjugation pilus retracts or depolymerizes when the two bacteria merge. A series of membrane proteins encoded by a conjugative plasmid forms a bridge and cleft between the two bacteria, now called a pair.

Using the rolling circle model of DNA replication, a nuclease cleaves a strand of plasmid DNA at the beginning of the transfer site (oT) of the plasmid and this jagged thread enters the recipient bacterium. The second strand remains in the donor cell. The donor and recipient plasmid strands make a complementary copy of themselves. Both bacteria now have a conjugative plasmid. This process is summarized in figure \(\PageIndex{6}\)).

(Video) 3.1.3 Phylogenetic Trees

This is the mechanism by which resistance plasmids (R plasmids),encoding resistance to multiple antibiotics and the formation of conjugative pilus are transferred from the donor bacterium to the recipient. This is a major problem in the treatment of opportunistic gram-negative infections, such as urinary tract infections, wound infections, pneumonia and septicemia caused by organisms such asE. coli, Proteus, Klebsiella, Enterobacter, Serratia, Gpseudomonas,as well as intestinal infections with organisms such asSalmonellaGshigella.

There is also evidence that the conjugation pilus may also serve as a direct conduit through which single-stranded DNA can be transferred during conjugation.

Abstract

1. Mutation is an alteration of gene function within a bacterium, and while it allows bacteria to adapt to a new environment, it happens relatively slowly.
2. Horizontal gene transfer allows bacteria to react much faster and adapt to their environment by taking over large DNA sequences from another bacterium in one go.
3. Horizontal gene transfer is a process by which an organism transfers genetic material to another organism that is not its descendant.
4. Mechanisms of bacterial horizontal gene transfer include transformation, transduction and conjugation.
5. In transformation, a piece of DNA from a dead and degraded bacterium enters a competent recipient bacterium and is exchanged for a piece of DNA from the recipient. Usually this includes similar strains of bacteria or strains of the same species of bacteria.
6. Transduction involves the transfer of chromosomal DNA fragments or plasmids from one bacterium to another by means of bacteriophages.
7. Conjugation is the transfer of DNA from a living donor bacterium to a living recipient bacterium through contact between cells. In gram-negative bacteria, it contains a conjugative pilus.
8. A conjugative plasmid is self-transferable, that is, it has conjugation genes known as tra genes that allow the bacterium to create a mating partner with another organism, and oriT (origin of transfer) sequences that determine where on the plasmid the transfer begins. . DNA transfer.
9. Mobilizable plasmids without autotransmissibility tragenes can be co-transferred into a bacterium possessing a conjugative plasmid.
10. Transposons ("jumping genes") are small pieces of DNA that code for enzymes that allow the transposon to move from one DNA site to another, either on the same DNA molecule or on a different molecule.
11. Conjugative transposons carry genes that enable the creation of conjugative pairs.
12. F⁺conjugation is transfer of F⁺plasmid carrying tra genes encoding only one pilus for conjugation and formation of mating pairs from the donor bacterium to the recipient bacterium. Mobilizable plasmids can be used during F⁺conjugation.
13. During Hfr conjugation, F⁺a plasmid containing the tra genes encoding pairing was inserted into the bacterial chromosome to form the Hfr bacterium. This results in the transfer of chromosomal DNA from the donor to the recipient, which can be replaced by a portion of the recipient's DNA through homologous recombination.