1. Siti Sarah Jumali 06-4832123 sarahjumali@ns.uitm.edu.my
Genetic Transfer
Siti Sarah Jumali

2. Overview on Bacterial Gene Transfer
Bacteria are usually haploid
Makes it easy to identify loss-of-function mutations in bacteria than in eukaryotes
These usual recessive mutations are not masked by dominant genes in haploid species
Bacteria reproduce asexually
Therefore crosses are not used in the genetic analysis of bacterial species
Rather, researchers rely on a similar phenomenon called genetic transfer
In this process, a segment of bacterial DNA is transferred from one bacterium to another

3. Genetic transfer
A process to transfer genetic material from a bacterium to another bacterium
Enhances genetic diversity
Confer resistance to antibiotic when one a antibiotic resistant bacterium transfer the gene to another bacterial cell

4. Mechanism of Gene Transfer
Conjugation
Direct physical interaction between Donor and recipient cell
Transduction
When virus infects a bacterium and transfer genetic material
Transformation
Information is taken from a dead bacterium which releases it to the environment

5. Mechanism of Gene Transfer

6. CONJUGATION

7. CONJUGATION
Direct physical interaction between Donor and recipient cell
E.g plasmid is transferred to a recipient cell from a donor
Requires the presence of a special plasmid called the F plasmid.

8. Conjugation cont’d
A “mating” process between a donor F+ (bacteria with fertility factor =plasmid) and an F- recipient cell.
Occurs in Gram - enteric bacteria like E.coli
Plasmids carry genes that are nonessential for the life of bacteria.
Uses pili (sex pilus).
Eg. plasmid replication enzymes.
Causes medical Problem: R-Factor = antibiotic resistance!

9. Conjugation
Discovered in 1946 in bacteria by Joshua Lederberg and Edward Tatum
They were studying strains of Escherichia coli that had different nutritional growth requirements
Auxotrophs cannot synthesize a needed nutrient
Prototrophs make all their nutrients from basic components
One auxotroph strain was designated bio– met– phe+ thr+
It required one vitamin (biotin) and one amino acid (methionine)
It could produce the amino acids phenylalanine and threonine
The other strain was designated bio+ met+ phe– thr–

11. The genotype of the bacterial cells that grew on the plates has to be bio+ met+ phe+ thr+
Lederberg and Tatum reasoned that some genetic material was transferred between the two strains
Either the bio– met– phe+ thr+ strain got the ability to synthesize biotin and methionine (bio+ met+)
Or the bio+ met+ phe– thr– strain got the ability to synthesize phenylalanine and threonine (phe+ thr+)
The results of this experiment cannot distinguish between the two possibilities

12. The need for physical contact
Bernard Davis later showed that the bacterial strains must make physical contact for transfer to occur
He used an apparatus known as U-tube
It contains at the bottom a filter which has pores that were
Large enough to allow the passage of the genetic material
But small enough to prevent the passage of bacterial cells
Davis placed the two strains in question on opposite sides of the filter
Application of pressure or suction promoted the movement of liquid through the filter

14. Many species of bacteria can conjugate
The term conjugation now refers to the transfer of DNA from one bacterium to another following direct cell-to cell contact
Many species of bacteria can conjugate
Only certain strains of a bacterium can act as donor cells
Those strains contains a small circular piece of DNA termed the F factor (for Fertility factor)
Strains containing the F factor are designated F+
Those lacking it are F–
Plasmid is the general term used to describe extra-chromosomal DNA
Plasmids, such as F factors, which are transmitted via conjugation are termed conjugative plasmids
These plasmids carry genes required for conjugation

16. Some info on plasmid
Small, circular pieces of DNA that are separate and replicate independently from the bacterial chromosome.
Contains only a few genes that are usually not needed for growth and reproduction of the cell.
But important in stressful situations
F plasmid, facilitates conjugation
Can give a bacterium new genes that may help for survival in changing environment.
Some plasmids can integrate reversibly into the bacterial chromosome.
An integrated plasmid is called an episome.

17. Plasmid There are several types of plasmids:
a. Conjugative plasmids – genes for sex pili and conjugation
b. Dissimulation plasmids – genes for enzymes that catabolize unusual organic molecules (Pseudomonas species – toluene, camphor, petroleum products)
c. Plasmids carrying genes for toxins or bacteriocins
d. Plasmids carrying genes for resistance (R) factors
i. Consist of two sets of genes – RTF (resistance transfer factor) and specific resistance genes (r-determinant)

18. Mechanism of Conjugation
The first step in conjugation is the contact between donor and recipient cells
This is mediated by sex pili (or F pili) which are made only by F+ strains
These pili act as attachment sites for the F– bacteria
Once contact is made, the pili shorten
Donor and recipient cell are drawn closer together
A conjugation bridge is formed between the two cells
The successful contact stimulates the donor cells to begin the transfer process

20. F’ factors can be transferred through conjugation
The result of conjugation is that the recipient cell has acquired an F factor
Thus, it is converted from an F– to an F+ cell
The F+ cell remains unchanged
In some cases, the F factor may carry genes that were once found on the bacterial chromosome
These types of F factors are called F’ factors
F’ factors can be transferred through conjugation
This may introduce new genes into the recipient and thereby alter its genotype

21. Hfr Strains
In the 1950s, Luca Cavalli-Sforza discovered a strain of E. coli that was very efficient at transferring chromosomal genes
He designated this strain as Hfr (for High frequency of recombination)
Hfr strains are derived from F+ strains

22. Mechanism in Hfr Strains

24. William Hayes demonstrated that conjugation between an Hfr and an F– strain involves the transfer of a portion of the Hfr bacterial chromosome
The origin of transfer of the integrated F factor determines the starting point and direction of the transfer process
The cut, or nicked site is the starting point that will enter the F– cell
Then, a strand of bacterial DNA begins to enter in a linear manner

25. The F– cell does pick up chromosomal DNA
It generally takes about hours for the entire Hfr chromosome to be passed into the F– cell
Most matings do not last that long
Only a portion of the Hfr chromosome gets into the F– cell
Since the nick is internal to the integrated F factor, only part of the plasmid is transferred and the F– cells does not become F+
The F– cell does pick up chromosomal DNA
This DNA can recombine with the homologous region on the chromosome of the recipient cell
This may provide the recipient cell with new combination of alleles

27. Hfr (High Frequency Recombination)
Hfr- bacterial plasmid integrates into the chromosome.
Medical Problem: Hfr antibiotic resistance genes are passed during binary fission (every time the cell divides). Therefore, antibiotic resistance spreads very rapidly!
When Hfr mate with F – bacteria, only the bacterial genes cross NOT plasmid genes.
Genetic diversity results in this case due to recombination.

29. Hfr (High Frequency Recombination)

30. Interrupted Mating Technique
Developed by Elie Wollman and François Jacob in the 1950s
The rationale behind this mapping strategy
The time it takes genes to enter the recipient cell is directly related to their order along the bacterial chromosome
The Hfr chromosome is transferred linearly to the F– recipient cell
Therefore, interrupted mating at different times would lead to various lengths being transferred
The order of genes along the chromosome can be deduced by determining the genes transferred during short matings vs. those transferred during long matings

31. Wollman and Jacob started the experiment with two E. coli strains
The donor (Hfr) strain had the following genetic composition
thr+ : Able to synthesize the essential amino acid threonine
leu+ : Able to synthesize the essential amino acid leucine
azis : Sensitive to killing by azide (a toxic chemical)
tons : Sensitive to infection by T1 (a bacterial virus)
lac+ : Able to metabolize lactose and use it for growth
gal+ : Able to metabolize galactose and use it for growth
strs : Sensitive to killing by streptomycin (an antibiotic)
The recipient (F–) strain had the opposite genotype
thr– leu– azir tonr lac – gal – strr
r = resistant

32. Wollman and Jacob already knew that
The thr+ and leu+ genes were transferred first, in that order
Both were transferred within 5-10 minutes of mating
Therefore their main goal was to determine the times at which genes azis, tons, lac+, and gal+ were transferred
The transfer of the strs was not examined
Streptomycin was used to kill the donor (Hfr) cell following conjugation
The recipient (F– cell) is streptomycin resistant

35. From these data, Wollman and Jacob constructed the following genetic map:
They also identified various Hfr strains in which the origin of transfer had been integrated at different places in the chromosome
Comparison of the order of genes among these strains, demonstrated that the E. coli chromosome is circular

36. TRANSDUCTION

37. TRANSDUCTION
The transfer of genetic material from donor bacteria to recipient bacteria via transducing agent (bacterial viruses called bacteriophage).
Discovered in 1952 by Zinder & Lederberg.
Two kinds of transduction:
generalized and
specialized.

38. Transduction
A bacteriophage is a virus that specifically attacks bacterial cells
It is composed of genetic material surrounded by a protein coat
It can undergo two types of cycles
Lytic
Lysogenic

40. It will switch to
the lytic cycle
Prophage can
exist in a dormant
state for a long
time
Virulent phages only
undergo a lytic cycle
Temperate phages can
follow both cycles

41. Transduction Phages that can transfer bacterial DNA include
P22, which infects Salmonella typhimurium
P1, which infects Escherichia coli
Both are temperate phages

42. Generalized transduction
Starts with the LYTIC CYCLE where a T- even phage infects E. coli killing the host cell, and synthesizing 2,000 copies of itself.
The T-even phage randomly packages bacterial DNA in a few defective phages.
Once a T –even phage infects another E. coli, this genetic information can be recombined into the host cell without causing the lytic cycle.
New genetic information is thereby transduced from one bacteria to another.

43. Generalized Transduction

44. Generalized Transduction

45. Specialized Transduction
Lambda phage infects E.coli but does not lyse the cell immediately. Instead it integrates into chromosome of the bacteria as a prophage and remains dormant.
This is called the LYSOGENIC CYCLE. Phage genes are replicated and passed to all daughter cells until the bacteria is under environmental stress, from lack of nutrients, etc.
Then phage gene will excise from the nucleoid and enter the LYTIC CYLE taking one adjacent gene for galactose metabolism.

46. Specialized Transduction cont’d
The gal transducing phage (lambda) makes ~ 2,000 copies of itself with the gal gene, and infects other E.coli.
When gal integrates into the nucleoid of other E. coli, it may provide these bacteria with a new capacity to metabolize galactose.

47. Specialized Transduction Graphic

48. Comparison of Bacteriophage
Comparison of bacteriophage transduction in E.coli.
Generalized Specialized
T even phage lambda phage
lytic cycle lysogenic
random packaging specific gal gene

50. TRANSFORMATION

51. TRANSFORMATION
The passage of homologous DNA from a dead donor cell to a living recipient cell.
Occurs in Streptococcus pneumoniae.
When S. pneumo dies the DNA can be absorbed by a living S. pneumo and recombined into the chromosome.
The gene for capsule formation is obtained in this way, as is a gene for penicillin resistance.
Discovered in 1929 by Fredrick Griffith.

52. Griffith’s Transformation Experiment

53. Griffith’s experiment
(a) Inject living encapsulated bacteria into mice, mice die, encapsulated bacteria isolated from dead mice. (b) Inject living nonencapsulated bacteria into mice, mice remain healthy, a few non-encapsulated bacteria can be isolated from the living mice – most phagocytized by leukocytes. (c) Inject heat-killed encapsulated bacteria into mice, mice remain healthy, no bacteria isolated from the living mice. (d) Inject living non-encapsulated and heat-killed encapsulated bacteria into mice, mice die, isolated encapsulated bacteria from dead mice.

55. The Experiments of Avery, MacLeod and McCarty
Avery, MacLeod and McCarty realized that Griffith’s observations could be used to identify the genetic material
They carried out their experiments in the 1940s
At that time, it was known that DNA, RNA, proteins and carbohydrates are major constituents of living cells
They prepared cell extracts from type IIIS cells containing each of these macromolecules
Only the extract that contained purified DNA was able to convert type IIR into type IIIS

57. Hershey and Chase Experiment with Bacteriophage T2
In 1952, Alfred Hershey and Marsha Chase provided further evidence that DNA is the genetic material
Inside the capsid
They studied the bacteriophage T2
It is relatively simple since its composed of only two macromolecules
DNA and protein
Made up of protein

58. Life cycle of the T2 bacteriophage

59. The Hershey and Chase experiment can be summarized as follows:
Used radioisotopes to distinguish DNA from proteins
32P labels DNA specifically
35S labels protein specifically
Radioactively-labeled phages were used to infect nonradioactive Escherichia coli cells
After allowing sufficient time for infection to proceed, the residual phage particles were sheared off the cells
=> Phage ghosts and E. coli cells were separated
Radioactivity was monitored using a scintillation counter

62. Transformation
The process by which a bacterium will take up extracellular DNA released by a dead bacterium
It was discovered by Frederick Griffith in 1928 while working with strains of Streptococcus pneumoniae
There are two types
Natural transformation
DNA uptake occurs without outside help
Artificial transformation
DNA uptake occurs with the help of special techniques

64. Transformation Graphic

65. TRANSPOSITION
Transposons (jumping genes) are big chunks of DNA that randomly excise and relocate on the chromosome.
Transposons were discovered in 1950 by Barbara McLintock in corn.
Causes antibiotic resistance in Staph. aureus, the famous methicillin resistant Staphlococcus aureus (MRSA) strain!

66. End of Slides