Transposons

Question 1

What are transposons?

Answer 1

Usually short segments of DNA that can move from one position in a DNA molecule to a new location in the same or different DNA molecule.

(AKA “Jumping Genes”)

Question 2

True or false: Transposons are autonomously replicating DNAs like a plasmid.

Answer 2

False, they are NOT.

Question 3

What type of mutagen can transposons function as?

Answer 3

An insertional mutagen

Question 4

How can transposons increase certain types of genome rearrangements in the bacterial chromosome like deletions and inversions?

Answer 4

Because multiple copies of a transposon in a chromosome can create two (or more) regions of homology for homologous recombination.

Question 5

True or false: Some transposons “carry” antibiotic resistance genes.

Answer 5

True, this makes transposons important genetic vectors for the rapid spread of antibiotic resistance among bacterial pathogens.

Question 6

What are the different types of transposons (mobile DNAs that encode a transposase)?

Answer 6

1. Insertion Sequences (IS elements)
2. Non-composite transposons (ex: Tn3)
3. Composite transposons (ex: Tn5)
4. Transposon chimeras

Question 7

What are other types of mobile DNAs (not including transposons)?

Answer 7

1. Invertible elements
2. Integrons
3. Genome Islands

Question 8

What are insertion sequences (IS elements)?

Answer 8

These are the smallest (about 1,000 bp) and simplest type of transposon. They usually only contain a single gene that codes for the enzyme transposase. This enzyme carries out all the chemical steps needed for transposition (i.e., to jump to a new location). The beginning and end of the IS DNA is defined by two inverted sequence repeats (about 20 bp) that mark the boundary of the transposon. Four different IS elements are known to occur in E. coli chromosomes (IS1, IS2, IS3, and IS4).

Question 9

What does the enzyme transposase do?

Answer 9

Carries out all the chemical steps needed for transposition (to jump to a new location)

Question 10

How can you tell what is the beginning and end of the IS DNA?

Answer 10

They are defined by two inverted sequence repeats (about 20 bp) that mark the boundary of the transposon.

Question 11

How many IS (insertion sequence) elements are found to occur in E. coli chromosomes?

Answer 11

There are 4:
1. IS1
2. IS2
3. IS3
4. IS4

Question 12

What are non-composite transposons?

Answer 12

Similar to IS elements but have additional genes besides encoding for transposase.

Question 13

Tn3 (an example of a non-composite transposon) contains an antibiotic resistance gene (bla) for what? Tn3 also contains a gene for what enzyme and sequence site?

Answer 13

Ampicillin resistance. The enzyme “resolvase” and sequence site called “res”.

Question 14

What is an example of a composite transposon?

Answer 14

Tn5

Question 15

What is Tn5?

Answer 15

A composite transposon composed of two IS elements (IS50 right and IS50 left) that make up the ends of the transposon (usually one of the IS elements is not functional). A large segment of DNA that lies between these two IS elements is then moved as one transposable unit. For Tn5, this intervening DNA contains antibiotic resistance genes.

Question 16

What antibiotic resistance genes can be found in Tn5 composite transposon?

Answer 16

Abx resistance for kanamycin, bleomycin, and streptomycin.

Question 17

What are transposon chimeras?

Answer 17

Transposons that also contain properties of other types of genetic elements. For example, the transposon Tn916 and Phage Mu.

Question 18

What is special about the transposon Tn916?

Answer 18

In addition to acting like a transposon, it will also act like a self-transmissible plasmid (conjugation plasmid), because Tn916 contains “tra” genes and an ori T site (in addition to transposase and tetracycline resistance). When Tn916 excises out of a donor DNA molecule it will form a circle and then act like a self-transmissible plasmid and will move from a donor cell to a recipient cell via conjugation.

Question 19

What is special about Phage Mu?

Answer 19

It is a lysogenic phage that contains a gene for transposase. The phage DNA acts like a transposon and inserts into random locations in the E. coli chromosome when it forms prophage DNA during lysogeny.

Question 20

What are invertible elements?

Answer 20

A type of mobile DNA that does not move to new locations in DNA. Rather these
elements simply stay in one location but can flip their orientation and can then flip back again.

Question 21

What is the “Hin” element?

Answer 21

A genetic element found in the chromosome of Salmonella responsible for a type of phase variation of the flagellin protein found in the flagellum (that is, a switch from producing a H1 type flagellin protein to a H2 type flagellin protein).

Question 22

What does the Hin element contain at each end of the element?

Answer 22

Inverted repeat sequences.

Question 23

What does the gene hin found in the Hin element code for?

Answer 23

A type of site-specific recombinase that acts only on the terminal repeat sequences.

Question 24

What are integrons?

Answer 24

Unusual genetic elements that “capture” a type of mobile DNA called a gene cassette.

Question 25

What three parts compose an integron?

Answer 25

1. a gene encoding an integrase (i.e., a site-specific recombinase)
2. a specific sequence site called att I that the integrase recognizes
3. adjacent to the att I site is a strong promoter called Pc

Question 26

What are gene cassettes?

Answer 26

Small cccDNAs (circular dsDNA) that cannot replicate autonomously. Often contain a single abx resistance gene, but this gene has no promoter and is thus not expressed from this gene cassette.

Question 27

What is att C (“59be”)?

Answer 27

A specific sequence found on gene cassettes which allows the integrase of the integron to capture the gene cassette via site-specific recombination between the att C sequence on the gene cassette and the att I sequence on the integron.

Question 28

What are super integrons?

Answer 28

Found in the chromosome of some bacteria and contain very large arrays of captured gene cassettes. Thought to be the source of gene cassettes.

Question 29

What are genome islands?

Answer 29

Large regions of DNA that can be up to 200,000 bp in size!

Question 30

Where are genome islands usually found?

Answer 30

In the chromosome, often adjacent to a tRNA gene.

Question 31

How are genome islands transferred into the chromosome?

Answer 31

Via transduction

Question 32

What are some examples of virulence genes that can be contained in genome islands?

Answer 32

Type III secretion systems and production of siderophores.

Question 33

What are the two different biochemical reactions that transposon elements that encode a transposase can use during transposition?

Answer 33

1. Replicative mechanism
2. Cut & Paste mechanism

Question 34

What is the replicative mechanism?

Answer 34

More complex reaction in which a copy of the transposon is produced during its movement from a donor DNA molecule to a target DNA molecule.

Question 35

What is the cut & paste mechanism?

Answer 35

There is no duplication of the transposon DNA when it jumps to the target DNA molecule. The fate of the donor DNA is not well understood.

Question 36

Why do most transposition events occur very infrequently?

Answer 36

Because transposons can produce insertional mutations when they want to jump to a new location in DNA. For this reason, most transposons contain mechanisms to keep the jumping event a rare occurrence.