Genetic Transfer

Siti Sarah Jumali

06-4832123

sarahjumali@ns.uitm.edu.my

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

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

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

Mechanism of Gene Transfer

CONJUGATION

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.

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!

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–

• 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

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

• 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

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.

Plasmid

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

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

• 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

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

Mechanism in Hfr Strains

• 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

• It generally takes about 1.5-2 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

•

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.

Hfr (High Frequency Recombination)

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

• 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

• 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

• 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

TRANSDUCTION

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.

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

Virulent phages onlyundergo a lytic cycle Temperate phages can

follow both cycles

Prophage canexist in a dormantstate for a longtime

It will switch tothe lytic cycle

Transduction

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

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.

Generalized Transduction

Generalized Transduction

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.

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.

S p ecialized T ran sd u ctio n G rap h ic

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

TRANSFORMATION

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.

Griffith’s Transformation Experiment

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.

• 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

The Experiments of Avery, MacLeod and McCarty

• In 1952, Alfred Hershey and Marsha Chase provided further evidence that DNA is the genetic material

Hershey and Chase Experiment with Bacteriophage T2

They studied the bacteriophage T2 It is relatively simple

since its composed of only two macromolecules

DNA and protein

Made up of protein

Inside the capsid

Life cycle of the T2 bacteriophage

• 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

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

• Transform

ation Graphic

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!

End of Slides