Transformation and transduction

Competence

• ability of a bacterial cell to take up extracellular DNA from the environment

• Can occur naturally, or be induced

Bacterial transformation

• inducing competence

Effect of taking up DNA

• increases genetic diversity

• Faster gene uptake than waiting to pass down generations

• Could uptake phage or viral DNA , leads to cell being killed

Requirement of transformation

• competent cells

When does competence occur

• just before entry to stationary phase , when growth stops

• Tries to improve fitness for survival

Nature of cell competence

• some species have a small number of cells become competent for hours

• Other species have all their cells become competent for a short period of time

Reason for competence at entry to stationary phase

• quorum sensing

• Depends on low nutrient levels

Quorum sensing

• ability to regulate genes based on population density

Mechanism for competence

• cells secrete ComX

• Inc cell density = inc ComX conc

• ComX binds to ComP (receptor)

• Signalling pathway leads to changes in gene expression where cells become competent

ComX

• small pheromone

CSF

• Pheromone

• Promotes competence and sporulation

Source of DNA for uptake

• DNA breaks upon lysis

What DNA do cells take up

• some cells will take up DNA of any origin

• Others will only take up DNA from the same species

How do species recognise their own DNA

• DNA contains recognition sequences

• Very common sequences , found in the fragments

Mechanism of uptake of DNA

• DNA binds to surface protein

• Enters cell

• If ss, cell makes it ds

• Binds to competence-specific protein

• Integrated into genome , mediated by RecA

Acquiring traits

• uptaking DNA can lead to cells acquiring new traits

transduction

• genetic exchange in bacteria

• Mediated by bacteriophage

Phage T2

• phage genome can be ss, ds , DNA or RNA

• Can be big or small

• Circular or linear

Phage ecology

• v abundant

• Not organisms, since dont have their own metabolism

• Co exist w hosts in all environments

What do phage influence

• characteristics of host

• Population dynamics

• Long term evolution

Antibiotic therapy

• broad spectrum, leads to resistance

• Poor on biofilms

• Can cause anaphylaxis or organ toxicity

Phage therapy

• Species or strain specific, issue if unknown source or multiple

• Works well on biofilms

• Generally safe, not well researched

PhageT2 lifestyle

• T2 infects and kills E. coli (lytic)

Lambda phage

• ds linear genome

• Tail interacts w E. coli

• Infection can be lytic or lysogenic

Lytic cycle (phage)

• attachment

• Insert genome

• New virions made

• Lysis, release of virions

• Cycle repeats

Restriction modification system

• restriction enzymes cleaves phage DNA

• Host DNA not cleaved because its methylated , many enzymes insensitive to methylation

• Phage DNA not methylated

Restrictive hosts

• restriction enzymes in host

• Reduce yields of virus

Lysogenic cycle

• attachment

• DNA inserted and integrated

• Stays in genome, transmitted to daughter cells unless lytic cycle triggered

Lysogen

• strain of bacteria carrying lysogenic phage

Prophage

• phage in lysogenic state

Suppression of lytic cycle in lysogen

• expression of a phage protein made by prophage

• Stops lysis

• The protein also suppresses lysis by other phage of the same type infecting the cell

• Carrying a prophage is an advantage to host

Lysogenic carrying other genes

• prophage can carry other genes

• Some genes may suppress transcription of the gene

• Repressor protein activated by substance, eg in deficiency the gene can be transcribed

• Leads to better survival of bacterial genome and the gene

• MOST PHAGES DONT CARRY DAMAGING GENES

Transduction

• transfer of DNA using phage vector

• Only some phages can transduce

• Only some bacteria are transducable

Generalised transduction

• transfer of any DNA to recipient cell by lytic or lysogenic phage

Specialised transduction

• transfer of specific genes via lysogenic phage

Generalised transduction mechanism

• infection

• Phage DNA replicate

• Host DNA degraded

• Virions packaged, occasionally contain bits of host DNA by mistake

• Virion inserts host DNA into recipient , DNA incorporated

• Virions defective, cant replicate since have no phage DNA

Specialised transduction mechanism

• DNA loops and cuts out phage DNA

• New phage DNA made

• Rare: excision of phage DNA carries the specific genes at integration site

• Defective virions made, missing part of phage genes

• Infect and transfer new gene into recipient cell , genes then integrate