Mobile DNA  Transposons By Anna Purna

Transposons  Transposons are the mobile segments of DNA which can jump or transpose from one place to another within the genome of a single cell.

Transposition  Transposition is the process of jumping of genes which is carried with the help of an enzyme known as transposase. This enzyme helps to make two single-stranded breaks in the target DNA.

What happens due to Transposition?  The mechanism of transposition can lead to phenotypically significant mutations. For example 1. Mice having almost identical DNA can exhibit a variation in their fur color. 2. Corn grains come in different colors (yellow, white or purple).

Characteristics of Transposons  Seen in almost all organisms but are relatively more abundant in eukaryotic organisms.  Eukaryotic genome consists of a large quantities of repetitive sequences and transpsosons can be the source of this repetitive DNA.  Transposons can stimulate structural, regulatory or both kinds of changes within the same organism.  They can adopt either of the two mechanisms which are “copy and paste" or "cut and paste”.

Discovery of Transposons  The jumping genes or transposons were discovered by Barbara McClintock and came into publication in the year She was also awarded a Nobel Prize in Medicine in the year 1983 for this discovery.

Classification of Transposons  Transposons are broadly classified into 3 classes which are as follows: 1. Insertion sequences or IS elements They are the smallest bacterial transposons which do not carry host genes. They carry only those genes which are needed for their own transposition. They have inverted repeats at the termini and carry the gene for transposase enzyme. They contain the signals for the abrupt chain termination or termination of the process of transcription very frequently.

IS Elements and Their Mechanism of Action

2. Composite Transposons  Also known as Class-I transposons or (retrotransposons).  An antibiotic resistance gene (or genes) is flanked by two IS elements.  Some times a terminal IS element can act as an individual transposable element.

Most Common Types of Retrotransposons 1. Long interspersed nuclear elements (LINEs) – They lack LTRs (long terminal repeats) and are transcribed by RNA polymerase II. They code for reverse transcriptase. 2. Short interspersed nuclear elements (SINEs) – They are transcribed by RNA polymerase III and do not code for reverse transcriptase.

Structure of Composite Transposon

3. Noncomposite Transposon  Also known as class-II transposons (DNA transpososns).  They are simple segments of DNA.  They range in length from 1,000 to as many as 40,000 base pairs  They contain a transposase gene, inverted repeats and several antibiotic resistance genes

Structure of Noncomposite Transposon

Mechanism of Transposition  There are two basic methods of transposition: 1. Conservative transposition:  This is seen with class II transposons which adopt a cut and paste mechanism  Transposase enzyme is generally seen within these elements and it cuts the transposon from the donor DNA (blunt cuts)

Mechanism of Transposition.. contd.  Transposase also cleaves target sites (through a staggered cut)  The transposon gets ligated into the target through transposase  DNA pol I and ligase help to fill the gaps by synthesizing new nucleotides.  During this process the donor DNA gets destroyed and the target DNA gets duplicated.

Direct repeats are produced within the Target DNA due to staggered cut:

2. Replicative Transposition  This mechanism is adopted by transposons belonging to class I and they adopt a copy and paste mechanism.  Requires transposase, resolvase and an IRS (internal resolution site) recognized by resolvase.  These transposons generate a second copy which gets inserted somewhere else. The original copy is left behind within the donor.  In the first step transposable DNA is copied into RNA.  RNA which jumps to a different location gets copied into DNA in the presence of reverse transcriptase.

2. Replicative Transposition.. contd.  This copied DNA gets inserted into a different location within the genome.  The Transposase generates the nicks and helps in the process of ligation.  Gaps during the process of ligation are filled by DNA pol I and ligase enzyme.  Resolvase catalyzes site specific recombination between the 2 IRSs.  This mechanism is involved in the spread of transposable elements within higher organisms.  Leads to the insertion of repetitive sequences throughout the genome.  The donor is not destroyed as the transposon does not leave it rather two copies of transposons are produced.

Mechanism of Replicative Transposition

Nomenclature of Transposons  Transposons are abbreviated as Tn followed by the numbers for identification. For example Tn1, Tn2, Tn3 etc.  Genes present on transposons are represented by their genotypic name for example if a transposon contains an ampicillin resistant gene it is denoted as Tn1(ampR).  Insertion sequences or IS elements were the first ones to be identified and they do not carry host genes. Hence they are labeled as IS1, IS2 etc.  When transposons are located within the gene: For example within the Lac gene, they are labeled with the help of gene name, allele number followed by two colons and the transposon present inside the gene ((like lacZ87::Tn3).

Scientific Applications of Transposons  Assist in the transfer of bacterial resistance to antibiotics. The genes for antibiotic resistance are generally carried by the transposable elements which later get incorporated into plasmids. Such plasmids can be used to transfer the genes of antibiotic resistance from one organism to another.

Applications of Transposons.. contd.  Transposons can lead to diseases. Transposons stimulate chromosomal rearrangements or gene mutations which might lead to diseases within human beings. Diseases generally associated with transposable elements are SCID, hemophilia A and B, Duchenne muscular dystrophy and porphyria.

Applications of Transposons.. contd.  Within animal cells transposons may effect the development of embryo and tumor formation.  Oncogenes get activated due to random reshuffling of transposons especially when they get located adjacent to the oncogenes.

Summary of the Class  Transposons are the jumping genes discovered by Barbara McClintock.  They require transposase enzyme for transposition.  Based on their structure and mechanism of action, they are broadly classified into three groups: IS, Composite and Non-composite Transposons.