The transposons present into the bacteria are DNA transposons contain transposase and antibiotic resistance coding genes.
IS, Tn5, Tn7, Tn10, Tn3 and Mu phage are some of the most studied transposons present into the bacteria.
The organization and behaviour of the eukaryotic transposons came in light after the study of the molecular structure of the bacteria transposons.
In the eukaryotes, the transposons can interfere with the gene expression, however, 99% of the transposable elements in human are now inactive.
The eukaryotic transposable elements are retrotransposons while the bacterial transposons are DNA transposons, mostly.
We had discussed the different types of transposons such as retrotransposons, LINEs, SINEs and LTR-retrotransposons in the previous article of this series. Read it here: Transposons: A Jumping Entity and a Foe with Benefits
Tn family elements and IS elements are commonly found in all type of bacteria.
If the TE contains the transposase gene, it confirms the antibiotic resistance otherwise remains as well.
In this present article, we are discussing only about the bacterial transposons and their role in the antibiotic resistance.
- What are the bacterial transposons?
- IS transposons
- Composite transposons
- Mu phage
- Transposase and antibiotic resistance
Let’s start the topic,
Abbreviation used in this article
|Type of transposons||Full name|
|LTR||Long Terminal Repeats|
|TSD||Target site duplication|
|TIR||Terminal Inverted Repeats|
|IS elements||Insertion sequence elements|
What are the bacterial transposons?
The transposons are potent enough to jump between different locations.
Structurally, the bacteria contains one unique bacterial chromosome and one circular plasmid DNA. The majority of the genetic material are located on the bacterial chromosome while the plasmid contains some of the important genes.
The plasmid contains several kb of DNA and some genes essential for their survival in some harsh environmental conditions.
Antibiotic resistance genes are located on the plasmid.
The transposable elements present into the bacteria can jump between plasmid and bacterial chromosomal DNA.
Types of transposons present in the bacteria based on their replicative efficiency are,
- Replicative transposons
- Non-replicative transposons
The replicative transposons, insert a copy of the transposon to another site through the process of replication.
The non-replicative or conservative transposons involve in the excision of the TE, it integrated into the new site by leaving the old site.
Insertion sequence transposons:
The IS elements are widely found in the bacterial genome both on a chromosome as well as on the extrachromosomal plasmid.
The IS elements are commonly present in almost all type of prokaryotic organisms.
Interestingly, the IS element contains genes only for the regulation and promotion of transposition which means it contains transposase as well as the resolvase.
In this later section of this article, we will discuss about the resolvase.
It is compactly organised, 1000 to 1500 nucleotides long gene comprising identical repeat sequences at both ends of it.
On both the ends, the IS element contains some of the identical or nearly identical repeat sequences. these repeats are located in inverted orientation and hence called an Inverted Terminal Repeats (ITRs) or Terminal Inverted Repeats (TIRs)
The inverted terminal repeats are 8 to 40 nucleotide pair long.
The Inverted Terminal Repeats are the main characteristic of all the IS elements present in all the prokaryotes.
Whether it is inserted into a bacterial chromosome or plasmid, it duplicates DNA at the site of insertion.
Then each copy of the DNA locates on two different ends of the transposon and these sequences are the direct repeat sequences. See the figure above it shows how the direct repeat situated on both sides of IS element.
The process is called a target site duplication.
After the terminal inverted repeats, the target site duplication sequences are located. The process of Target Site Duplication is shown in the figure below,
As shown into the figure above, during the target site duplication, a staggered break occurs both the side of the DNA. The Insertion sequence inserts between the gap.
After that, the breaks on both the side are filled or repaired by the process of duplication. At the end of the TSD process, the direct repeats of the host DNA generate on both the end of the IS element. Examine the above figure carefully.
As the IS element does not carry any other DNA sequences, the only DNA segment present between two TIRs codes for the proteins which helps in the insertion only.
The E.coli IS elements are categorised into IS-1, IS-2, IS-3, IS-,4 and IS-5. The IS elements comprise only 0.3% of total genomic composition of bacterial DNA.
However, multiple copies of IS elements present into the bacterial cell.
|IS elements||Size in (bp)||No. of copies/ genome|
The composite transposons are a combination of two IS elements.
When two IS elements inserted near to each other it creates the composite transposon. It clearly indicates that this type of TE contains two copies of the same type of IS elements.
It flanking a fragment of DNA containing one or even more genes that encode for some protein (proteins other than transposase and resolvase).
These are mostly antibiotic-resistant genes, uses the conservative manner of transposition.
The Tn5 elements contain 2 copies of IS50 elements on both the flanking regions and antibiotic resistance genes.
Interestingly, here, the elements present on both the flanking regions are the same but are not identical.
The structure of the Tn5 composite element is shown into the figure below,
The Tn5 contains the IS50 elements in which the left flanking element is named as IS50L and the right side IS50 are named as IS50R.
The IS50L does not contain the transposase gene hence it cannot move while the IS50R contains extra sequences for the transposase gene which makes it capable of moving from one position to another.
The transposition activity of the composite transposons is highly regulated and this is the main characteristic of the Tn5 (composite transposons).
How the activity of composite transposons are regulated?
The composite transposons in bacteria contain the genes for Resolvase and beta-lactamase, besides transposase.
Due to the presence of the IS elements on both the flanking region, the composite elements contain terminal repeat sequences.
The resolvase is the repressor of the transposition for the composite elements. It decreases the production of transposase and so decreases the frequency of transposition.
Tn9 and Tn10 are other examples of the composite transposons. The Tn5, Tn9 and Tn10 carry the kanamycin, chloramphenicol and tetracycline resistance genes, respectively.
Some of the other examples of composite transposons:
Read more articles:
Some of the transposons do not contain IS elements on both ends of it called non-composite transposons.
Even though it does not have the IS elements, it contains inverted repeats on both the ends of it.
Tn3 is the best example of non-composite TE.
It contains 30-40 nucleotide long terminal inverted repeats.
The middle sequences of the Tn3 encode for the ampicillin resistance gene which is nearly ~5kb long.
Structurally, the Tn3 elements contain tnpA, tnpB and bla gene which encoding transposase, resolvase and beta-lactamase, sequentially. See the figure below,
The two gene tnpA and tnpB are responsible for the regulation of transposition while the bla gene confirms resistance against the ampicillin. The Tn3 has the 38np terminal repeats on both the ends.
Likewise the composite TE, the non-composite TE has the transposase and resolvase coding genes too.
“IS elements, composite transposones and non-composite transposons causes duplication of target site. “
The phages are also considered as a transposon because it can integrate into the bacterial genome randomly. Mu phage is the best example of replicative transposons also named as transposable phages.
It is a mutator hence named as “Mu phage”.
It has many common features with the IS elements of the bacteria. However, it is larger than the IS elements (36kb).
The general structure of the Mu phage is shown in the figure above.
It can causes mutation by inserted in any orientation into the plasmid however, it can be reverted.
Structurally, the Mu phase contains the Inverted repeat sequences and flanking DNA sequences from the previous host.
Besides this, the coding sequence of the Mu phase contains genes for replication, coat protein and lysis.
Here one of the important points that make the phage Mu different from the IS elements is that the flanking DNA sequences present on either side of the Mu are not been inserted into the new host sequence.
Additionally, the insertional sequences are not present on the terminal ends of it likewise the IS elements.
The Mu phase TE is capable of transferring DNA sequences and other phage DNA into the host genome.
For that, two Mu elements combined together and mobilize the DNA in between. Further, Mu mediated transposition is possible for F-factor, bacterial markers and resistance genes between bacterial chromosome and plasmid.
- PCR reaction: Ten secrets that nobody tells you
- Importance of Tris-EDTA (TE) buffer in DNA extraction
Now coming to the main point of our present article,
How the antibiotic resistance help bacteria to survive and how it is developed through the transposition?
Transposase and antibiotic resistance:
A plasmid in bacteria contains some of the genes for the survival of their own. It also contains an antibiotic resistance gene or genes for multiple antibiotic resistance.
The antibiotic resistance genes are the class of genes which give power to the bacterium for surviving even in the presence of the antibiotic or drug.
If the resistance gene is carried by the transposon, eventually it is copied and inserted into another plasmid.
Due to the continuous random transposition, the antibiotic resistance gene spreads so quickly between the bacterial colonies through the plasmid.
Now let’s understand how the transposon mediates the transposition of the antibiotic resistance gene.
“Antibiotic resistance is one of the emerging problems that might be the biggest threat in the future.”
We will understand the mechanism by taking the example of the transposition of Tn3.
The plasmid comprises the Tn3 TE, let’s name it as a donor, therefore, the plasmid which will receive the transposon is a recipient.
In the very first step, through the process of cointegration, a cointegrate is formed due to the fusion of two plasmid.
During this process, another Tn3 element is originated by replication in the same orientation.
In the second step, the site-directed recombination occurs at the “res” site. The “res” is the resolution site at which the resolvase controls the recombination of the transposon. See the figure below,
Here the transposon (in this case Tn3) contains two genes one for the transposase and another for the resolvase, the trpA and trpB respectively.
The activity of both the genes are dependent on each other, the transposase overcome the activity of repressor or resolvase for transposition,
While the resolvase (which is a repressor) prevents the activity of the transposase by controlling the overall transposition process.
The “res” site is the specific location located between the trpA and trpB gene. Therefore, the product of the gene trpB the resolvase repress the synthesis of transposons.
Because of this reason, it is also called a regulator of the transposition in the bacteria.
After the completion of recombination, two plasmid molecules each with a copy of transposon are generated.
The antibiotic resistance gene cannot be moved without the transposase enzyme, the plasmid having it, is known as R-plasmid.
The segment on the R plasmid carrying the transposase specific gene is called a resistance transfer factor, RTF.
The resistance gene, transposase and the flanking sequence of the transposon are same in all R-plasmids (however, the resistance gene may vary) and called as the R- determinant.
The presence of multiple IS elements or the composite transposons are responsible for the development of the multiple drug resistance.
The transposase mediates all the activity hence development of the antibiotic resistance in bacteria is govern by the transposase.
After discussing all these, an obvious question arises in mind,
Why antibiotic resistance is a threat to us?
First, take a look at this case study from Japan,
The Japanese scientist collected samples from streams, polluted lakes and sewers and performed a meta-experiment on the bacteria present on those samples.
They had concluded that, in less than 10 years, the R-plasmid mediated antibiotic resistance is increased 80% higher than the original frequency in all bacteria species present into those samples.
The evolution and rapid outspread of R-plasmid is one of the biggest challenges to the medical fraternity. The R-plasmid spreads not only between the species but also dispersed across different species of bacteria.
The transposition of E.coli R-plasmid is found in some of the genera such as Salmonella, Hemophilus, Shigella, Proteus and Pasturella.
Some of the transposons that carry the antibiotic resistance:
Besides E.coli these all are now resistance to the antibiotics such as penicillin, tetracycline, streptomycin and kanamycin.
Therefore, using antibiotics for every minor infection is not a good practice, we have to restrict the use of the antibiotics otherwise, the antibiotics that are so effective today may not work against that particular infection in the future.
Our other articles related to this topic:
- Types of it- LINEs, SINEs, Retrotransposons- Importance of TE.
The bacterial transposons are still active in nature and it gives power to them. IS, Tn5, Tn3, Tn9 and Tn10 are now actively involved in the development of new antibiotic resistance plasmids in bacterial species.
Antibiotic resistance is one of the serious problems in the upcoming years. Use the antibiotics wisely and complete the course given by the doctor.