Notes
Bacterial Horizontal DNA Transfer
Sections
Bacterial Horizontal DNA Transfer
Three mechanisms of DNA transfer between cells:
Conjugation, which typically involves the unidirectional transfer of plasmid DNA.
Transduction, which occurs when bacteriophages transfer DNA between cells.
Transformation, which occurs when bacteria take up foreign DNA and incorporate it into their own.
Conjugation
First Drawing:
Donor cell has circular F plasmid (F stands for "fertility factor").
Recipient cell, which does not have an F plasmid.
F+ cell extends its sex pilus, aka, conjugative pilus, and attaches to the F- cell.
This brings the two cells in close physical contact.
Second Drawing:
One strand of the F plasmid unwraps and enters the recipient cell.
Third Drawing:
Each cell now has a single strand of the original F plasmid.
Fourth Drawing:
Each cell synthesizes a complementary strand of plasmid DNA, so that both cells are now F+.
High-frequency recombination (Hfr)
Occurs when the plasmid is integrated into the bacterial DNA; when the plasmid is transferred to a new recipient cell, it takes with it some of the donor cell's DNA.
Resistance plasmids
Carry genes that confer antibiotic resistance to bacterial cells; conjugation allows quick dissemination of this trait throughout a colony, ensuring its survival despite antibiotic drugs.
Transduction
There are two types:
Generalized, which involves the transfer of non-specific genes.
Specialized, which involves the transfer of specific genes.
We show an example of generalized transduction.
First Drawing:
Bacterial cell and with its DNA.
A bacteriophage, aka, bacterial virus, injects its DNA into the bacterial cell.
Second Drawing:
Within the bacterial cell, the phage replicates, and the bacterial DNA is fragmented.
Third Drawing:
As the phage replicates and assembles, some phages will incorporate bacterial DNA fragments.
Fourth Drawing:
Ultimately, bacterial cell lysis releases these phages.
Fifth Drawing:
Phages with the bacterial DNA can inject the donor bacterium's DNA into a new recipient.
Within the new cell, the donor DNA combines with the recipient's DNA, forming recombinant DNA.
Transformation
First Drawing:
The donor cell with its bacterial DNA.
Second Drawing:
When the bacterium lyses, it releases DNA fragments.
These fragments can then enter recipient cells.
Third Drawing:
Fragments are integrated into recipient DNA.
Some bacteria, such as Neisseria, have surface receptors to facilitate transformation. Frequent transformation may facilitate immune system and/or antibiotic evasion.
Transfection
Scientists use transformation to study gene expression or inhibition by artificially injecting foreign DNA into a eukaryotic cell nucleus, a process called transfection; scientists can also use viruses in a form of artificial transduction.
DNA transfer to Eukaryotes
Agrobacterium tumefaciens is a bacterium that causes crown gall disease in plants.
It does so by transferring the Ti (tumor-inducing) plasmid DNA, which, as its name suggests, leads to tumor development in the infected plant.
Scientists harness this natural phenomenon to study gene effects in the lab; they can insert additional genes into the plasmid to produce transgenic, aka, genetically modified, plants.
Full-Length Text
- Here we will learn about horizontal, aka, lateral DNA transfer, which involves the transfer of genetic information between cells.
- It is thought that horizontal DNA transfer enables bacteria to adapt to changes in their environment, within a single generation.
- To begin, denote three mechanisms of DNA transfer between cells:
- Conjugation, which typically involves the unidirectional transfer of plasmid DNA;
- Transduction, which occurs when bacteriophages transfer DNA between cells;
- And, Transformation, which occurs when bacteria take up foreign DNA and incorporate it into their own.
Let's begin our diagram with conjugation.
- First, draw a donor cell, which contains the circular F plasmid (F stands for "fertility factor");
- Indicate the nucleoid region with bacterial DNA.
- Then, show the recipient cell, which does not have an F plasmid.
- Now, indicate that the F+ cell extends its sex pilus, aka, conjugative pilus, and attaches to the F- cell.
- This brings the two cells in close physical contact.
- Next, show that one strand of the F plasmid unwraps and enters the recipient cell.
- As a result, each cell now has a single strand of the original F plasmid.
- Finally, show that each cell synthesizes a complementary strand of plasmid DNA, so that both cells are now F+.
- Denote that high-frequency recombination (Hfr) occurs when the plasmid is integrated into the bacterial DNA; when the plasmid is transferred to a new recipient cell, it takes with it some of the donor cell's DNA.
- Plasmids can also contribute to the survival and spread of bacteria:
- Denote that resistance plasmids carry genes that confer antibiotic resistance to bacterial cells; conjugation allows quick dissemination of this trait throughout a colony, ensuring its survival despite antibiotic drugs.
Next, let's learn transduction.
- Denote that there are two types:
- Generalized, which involves the transfer of non-specific genes, and,
- Specialized, which involves the transfer of specific genes.
Let's illustrate an example of generalized transduction.
- To begin, draw a bacterial cell and indicate its DNA.
- Then, show that a bacteriophage, aka, bacterial virus, injects its DNA into the bacterial cell.
- Within the bacterial cell, the phage replicates, and the bacterial DNA is fragmented.
- Then, show that as the phage replicates and assembles, some phages will incorporate bacterial DNA fragments.
- Ultimately, bacterial cell lysis releases these phages.
- Phages with the bacterial DNA can infect new bacterial cells and inject the donor bacterium's DNA into the recipient.
- Within the new cell, the donor DNA combines with the recipient's DNA, forming recombinant DNA.
Finally, let's show transformation.
- First, draw the donor cell with its bacterial DNA.
- When this cell lyses, it releases DNA fragments.
- These fragments can then enter recipient cells and integrate into their DNA.
- Write that some bacteria, such as Neisseria, have surface receptors to facilitate transformation.
- Frequent transformation may facilitate immune system and/or antibiotic evasion.
- Scientists use transformation to study gene expression or inhibition by artificially injecting foreign DNA into a eukaryotic cell nucleus, a process called transfection; scientists can also use viruses in a form of artificial transduction.
So far, we've only discussed horizontal gene transfer in the context of bacteria; let's consider an example of DNA transfer from bacteria to eukaryotic organisms.
- Indicate that Agrobacterium tumefaciens is a bacterium that causes crown gall disease in plants.
- It does so by transferring the Ti (tumor-inducing) plasmid DNA, which, as its name suggests, leads to tumor development in the infected plant.
- Scientists harness this natural phenomenon to study gene effects in the lab; they can insert additional genes into the plasmid to produce transgenic, aka, genetically modified, plants.
References:
- Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. Medical microbiology. Philadelphia: Elsevier/Saunders. (2013).
- Levinson, W. E. Review of Medical Microbiology and Immunology. 14th Ed. Lange (2016)
- Bronesky, D., Wu, Z., Marzi, S., Walter, P., Geissmann, T., Moreau, K., Vandenesch, F., et al. 2016. Staphylococcus aureus RNAIII and its regulon link quorum sensing, stress responses, metabolic adaptation, and regulation of virulence gene expression. Annu. Rev. Micriobol. 70:299-316.
- O'Donnell, M., Langston, L., Stillman, B. 2013. Principles and concepts of DNA replication in Bacteria, Archaea, and Eukarya. Cold Spring Harb Perspect Biol. 5:a010108.
- Sieber, K. B., Bromley, R. E., Hotopp, J.C.D. 2017. Lateral gene transfer between prokaryotes and eukaryotes. Experimental Cell Research 358:421-426.
- Croucher, N. J., Mostowy, R., Wymant, C., Turner, P., Bentley, S. D. Fraser, C. 2016. Horizontal DNA transfer mechanisms of bacteria as weapons of intragenomic conflict. PLoS Biol 14(3): e1002394.
- Wendel, B.M., Courcelle, C.T., Courcelle, J. 2014. Completion of DNA replication in Escherichia coli. PNAS 111(46): 16454-16459.
- Yao, N. & O'Donnell, M. 2016. Bacterial and eukaryotic replisome machines. JSM Biochem Mol Biol. 3(1).
Images:
- Crown Gall Root (Wikipedia; Author: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org)