Transduction

Transduction and cotransduction

1. TRANSDUCTION Dr. M. Sonia Angeline Assistant Professor Kristu Jayanti College

2. INTRODUCTION Transduction is a mode of genetic transfer from one bacterium to another through a virus.   There is no direct contact between the bacterial cells.   Transduction is a method of gene transfer in bacteria from donor to recipient using bacteriophage. In this process, bacteriophages, which infect bacteria, use host cells to multiply and while assembling they sometimes pack the bacterial DNA with them. Later , when these viruses/ bacteriophages infect new bacterial cells, the bacterial genome that they carry may get inserted into the host genome. Transduction is commonly used in genetic engineering for inserting foreign DNA into the host cell. Transduction was discovered by Zinder and Lederberg in  Salmonella . Hershey and Chase used transduction as a tool to confirm that DNA is the genetic material.

3. STEPS INVOLVED IN TRANSDUCTION: Infection of the bacterial cell by bacteriophage. The virus uses the host machinery to make multiple copies either directly by the lytic cycle or first gets incorporated into the bacterial genome by the lysogenic cycle and followed by the lytic stage. During assembly of bacteriophages, the bacterial genome also gets packed by mistake in the viral head alongside the viral genome. In the lysogenic cycle, during excision of prophage, some parts of the bacterial genome that flank the prophage are also excised and go inside the assembled viral head together with the viral genome.

4. STEPS INVOLVED IN TRANSDUCTION: When these viruses infect another bacterial cell, they inject the viral DNA as well as donor DNA into the host cell. The bacterial DNA either forms plasmids or gets inserted into the recipient DNA if it is homologous to the recipient genome. Most of the time it remains as an extrachromosomal DNA. It can also get inserted with the prophage if it is a temperate phage. So the fate depends on the portion of bacterial DNA and also on the nature of bacteriophages.

5. GENERALIZED TRANSDUCTION If all the fragments of donor DNA from any region of chromosome have a chance to enter into transducing bacteriophage then it is known as generalized transduction. In this type of transduction, at first bacteriophage infects donor cell and begins lytic cycle. When virus enter into bacterial cell, virus hijack host cell and synthesize virus components such as genome, enzymes, capsid, head tail and tail fibers. Then viral enzyme hydrolyses host cell DNA into small fragments. During assembly of virus component to form progeny viruses, sometime any of the fragments of donor DNA get incorporated into the virus capsid (bacteriophage head). Such abnormal bacteriophage when infects a new cell, it can transfer this donor DNA into new bacteria. Since this donor DNA is not viral DNA, it does not replicates inside recipient bacteria but undergoes homologous recombination with recipient cell’s chromosomal DNA forming recombinant cell.

6. SPECIALIZED TRANSDUCTION In specialized transduction, bacteriophage transfer only a few restricted gene (DNA fragments) from donor bacteria to recipient bacteria. Specialized transduction is carried only by temperate bacteriophage which undergoes lysogenic cycle in donor cell. At first temperate bacteriophage enter into donor bacteria and then its genome gets integrated with host cell’s DNA at certain location and remains dormant and pass generation to generation into daughter cell during cell division. The bacteriophage which follows lysogenic cycle is known as temperate phage. When such lysogenic cell is exposed to certain stimulus such as some chemicals or UV lights, it causes induction of virus genome from host cell genome and begins lytic cycle.

7. SPECIALIZED TRANSDUCTION On induction from donor DNA, this phage genome sometimes carries a part of bacterial DNA with it. The bacterial DNA lies on sides of integrated phage DNA are only carried during induction. When such bacteriophage carries a part of donor bacterial DNA infects a new bacteria, it can transfer that donor DNA fragments into new recipient cell. So , in this specialized transduction only those restricted gene are situated on the side of integrated viral genome have a chance to enter into recipient cell.

8. CO TRANSDUCTION Transduction can also be used to map the relative locations of different bacterial genes on the chromosome. The principle is simply that the amount of DNA carried by any one transducing particle represents a very small portion of the total DNA of the bacterial cell . Transduction can also be used to map the relative locations of different bacterial genes on the chromosome. The principle is simply that the amount of DNA carried by any one transducing particle represents a very small portion of the total DNA of the bacterial cell. Any two genes that can be contained within a single transducing particle (meaning they can be " cotransduced ") must, therefore, be relatively close to each other on the chromosome.

9. CO TRANSDUCTION By measuring the cotransduction frequency of two or more genes, one can estimate the relative proximity of those genes. The simplest analysis is to select for the transfer of one gene (for example, a gene encoding antibiotic resistance), then to screen the population of transductant cells for the inheritance of a second gene. The cotransduction frequency is measured as the percentage of cells that have inherited both genes, rather than the selected first gene by itself. The higher their cotransduction frequency, the closer two genes must be to each other.

10. Genetic mapping chromosome transfer by Hfr strains Due to  the F factor's inherent tendency to transfer itself during conjugation , the rest of the bacterial genome is dragged along with it. Therefore , unlike a normal F +  cell, Hfr strains will attempt to transfer their entire DNA through the mating bridge, in a fashion similar to the normal conjugation.

11. Hfr strains A strain of bacterial that possesses the F factor integrated into the bacterial genome , hence, when it conjugates with another bacterium, it attempts to transfer a copy of the F factor as well as a portion of or the entire chromosome to the recipient bacterium . Therefore, we can immediately determine the order of the genetic markers simply by looking at the percentage of recombinants for any marker among the leu +  recombinants. Because the inheritance of thr +  is the highest, this must be the first marker to enter after leu . The complete order is  leu , thr , ile , mal, trp.

12. Co Transduction for Mapping Cotransduction can be used to determine a genetic map of the relative distances between genetic markers and the order of the markers. The use of cotransduction to determine gene order is based on the fact that the closer genes are to each other, the greater the probability they will be cotransduced . That is, the frequency of cotransduction is inversely proportional to the distance between two genes.

13. Genetic Mapping in bacteria by transduction Transductants - once the donor genetic material has been introduced into the recipient. It may undergo genetic recombination with a homologous region of the recipient chromosome. The recombinant recipient is called transductants . Phage Lysate- the suspension of released progeny phages is called as a phage lysate.

14. Genetic Mapping in bacteria by transduction Generalized transduction can be used to make linkage map and also to find gene order and map distance between co-transduced genes. Selected markers - a marker gene which help in the selection of transduced cells among the population of transduced and non transduced cells . Unselected markers- rest other markers are considered as unselected markers.

15. E. coli genetic map Two principal techniques of genetic mapping will be mentioned in the following discussion. The first method is mapping by "time of entry," in which genetic markers are positioned on the chromosome by determining the time when each marker first enters a recipient cell during the course of chromosome transfer from an Hfr donor strain. The second, and generally more accurate, mapping technique relies on transduction crosses mediated by bacteriophage P1 of E. coli. Two-factor transduction crosses provide estimates of the distance between pairs of markers separated by not more than 1.5 to 2 min by making use of the empirical "minutes-to-per cent frequency of cotransduction ". Multifactor transduction crosses involving three or more cotransducible markers permit precise determinations of the sequence of markers with respect to each other and to the linkage map as a whole. Groups of closely linked markers for which the gene sequence relative to the whole map is known are referred to in this review as having a known orientation.