Transcript Genetic-Exchange - Microbiology and Immunology Online
Exchange of Genetic Information Dr. Jeffrey Patton Associate Professor Pathology, Microbiology, and Immunology USC-School of Medicine
TEACHING OBJECTIVES: 1. To explain the mechanisms of gene transfer in bacteria.
2. To describe the nature of transposable genetic elements and plasmids.
3. To discuss the significance of gene transfer, transposable genetic elements and plasmids.
Mutations in Bacteria • Mutations arise in bacterial populations – Induced – Spontaneous • Rare mutations are expressed – Bacteria are haploid – Rapid growth rate • Selective advantage enriches for mutants • Gene transfer occurs in bacteria
General Features of Gene Transfer in Bacteria • Unidirectional – Donor to recipient • Donor does not give an entire chromosome – Merozygotes • Gene transfer can occur between species
Transformation • Definition: Gene transfer resulting from the uptake of DNA from a donor.
• Factors affecting transformation – DNA size and state • Sensitive to nucleases – Competence of the recipient (
Bacillus, Haemophilus, Neisseria, Streptococcus
) • Competence factors • Induced competence
Transformation
• Steps – Uptake of DNA • Gram + • Gram – Recombination • Legitimate, homologous or general • recA, recB and recC genes • Significance – Phase variation in
Neiseseria
– Recombinant DNA technology
Transduction • Definition: Gene transfer from a donor to a recipient by way of a bacteriophage • Bacteriophage (phage): A virus that infects bacteria
Phage Composition and Structure • Composition – Nucleic acid • Genome size • Modified bases – Protein • Protection • Infection
Contractile Sheath
• Structure (T 4 ) – Size (80 X 100 nm)
Tail Fibers
– Head or capsid – Tail
Head / Capsid Tail Base Plate
Infection of Host Cells by Phages • Adsorption –Tail fibers – Receptor is LPS for T4 • Irreversible attachment – Base plate • Sheath Contraction • Nucleic acid injection • DNA uptake
Microbe Library, American Society for Microbiology www.microbelibrary.org
Types of Bacteriophage • Lytic or virulent – Phage that multiply within the host cell, lyse the cell, and release progeny phage (
e.g.
T4) • Lysogenic or temperate phage: Phage that can either multiply via the lytic cycle or enter a quiescent state in the bacterial cell. (
e.g.,
l ) – Expression of most phage genes repressed – Prophage – Phage DNA in the quiescent state – Lysogen – Bacteria harboring a prophage
Events Leading to Lysogeny • Circularization of the phage chromosome – Cohesive ends
Cohesive Ends Ligase Linear Double Stranded Opened Circle Closed Circle
Events Leading to Lysogeny • Site-specific recombination requires – Phage coded enzyme (Int, integrase) – Bacterial encoded IHF (Integration Host Factor)
gal
• Repression of the phage genome (maintains lysogeny) – Repressor protein (cl) – Specific – Immunity to superinfection by other l because of promoter repression
gal bio gal bio bio
Termination of Lysogeny • Induction – Adverse conditions (DNA damage, ie UV) • Role of proteases – recA protein is activated – Destruction of repressor cI
gal gal bio bio gal bio
• Gene expression (repression lifted) • Excision • Lytic growth
gal bio
Transduction • Definition: Gene transfer from a donor to a recipient by way of a bacteriophage • Resistant to environmental nucleases
Transduction • Types of transduction – Generalized - Transduction in which potentially any donor bacterial gene can be transferred
Generalized Transduction • Infection of Donor • Phage replication and degradation of host DNA • Assembly of phages particles • Release of phage • Infection of recipient • Homologous recombination Potentially any donor gene can be transferred
Transduction • Types of transduction – Generalized - Transduction in which potentially any dornor bacterial gene can be transferred.
– Specialized - Transduction in which only certain donor genes can be transferred
Specialized Transduction Lysogenic Phage • Excision of the prophage • Replication and release of phage • Infection of the recipient • Lysogenization of the recipient – Homologous recombination also possible
gal gal gal bio bio gal bio bio bio
Transduction • Definition • Types of transduction • Significance – Common in Gram+ bacteria – Lysogenic (phage) conversion • e.g.
Corynebacterium diptheriae
–
Toxin derived from lysogenic phage
toxin
Conjugation • Definition: Gene transfer from a donor to a recipient by direct physical contact between cells • Mating types in bacteria – Donor • F factor (Fertility factor) –
F (sex) pilus
– Recipient • Lacks an F factor
Dono r Recipient
Physiological States of F Factor • Autonomous (F + ) – Characteristics of F + crosses x F • F becomes F + while F + remains F + • Low transfer of donor chromosomal genes
F+
Physiological States
of F Factor
Integrated (Hfr) (High Frequency of Recombination) – Characteristics of Hfr x F crosses • F rarely becomes Hfr while Hfr remains Hfr • High transfer of certain donor chromosomal genes
F + Hfr
Physiological States of F Factor • Autonomous with donor genes (F’) – Characteristics of F’ x F crosses • F while F’ remains F’ becomes F’ • High transfer of donor genes on F’ and low transfer of other donor chromosomal genes
Hfr F’
Mechanism of F + x F Crosses • Pair formation – Conjugation bridge • DNA transfer – Origin of transfer – Rolling circle replication
F + F + F F + F + F + F F +
Mechanism of Hfr x F Crosses • Pair formation – Conjugation bridge • DNA transfer – Origin of transfer – Rolling circle replication • Homologous recombination
Hfr Hfr F F Hfr Hfr F F -
Microbe Library, American Society for Microbiology www.microbelibrary.org
Mechanism of F’ x F Crosses • Pair formation – Conjugation bridge • DNA transfer – Origin of transfer – Rolling circle replication
F’ F F’ F’ F’ F’ F F’
Conjugation
• Significance – Gram - bacteria • Antibiotic resistance • Exponential increase under selective pressure – Gram + bacteria • Production of adhesive material by donor cells
Transposable Genetic Elements • Definition: Segments of DNA that are able to move from one location to another • Properties – “Random” movement – Not capable of self replication (not a replicon) – Transposition mediated by site-specific recombination • Transposase – Transposition may be accompanied by duplication
Types of Transposable Genetic Elements • Insertion sequences (IS) – Definition: Elements that carry no other genes except those involved in transposition – Nomenclature - IS1 – Structure (flanking inverted repeats) – Importance
ABCDEFG
• Insertional Mutation •Plasmid insertion •Phase variation
Transposase GFEDCBA
Phase Variation in
Salmonella
H Antigens H1 gene IS H2 gene H1 flagella H2 flagella
Types of Transposable Genetic Elements • Transposons (Tn) – Definition: Elements that carry other genes in addition to those involved in transposition – Nomenclature - Tn10 – Structure • Composite Tns
IS IS Resistance Gene(s) Resistance Gene(s) IS IS
– Importance • Antibiotic resistance
Plasmids • Definition: Extrachromosomal genetic elements that are capable of autonomous replication (replicon) • Episome - a plasmid that can integrate into the chromosome
Classification
of
Plasmids
• Transfer properties – Conjugative – Nonconjugative • Phenotypic effects – Fertility – Bacteriocinogenic plasmid – Resistance plasmid (R factors)