Download 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
Head/Capsid
• Genome size
• Modified bases
– Protein
• Protection
• Infection
• Structure (T4)
– Size (80 X 100
nm)
– Head or
capsid
Contractile
Sheath
Tail
Tail
Fibers
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)
• 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
gal
bio
Termination of Lysogeny
• Induction
– Adverse conditions
(DNA damage, ie UV)
• Role of proteases
– recA protein is activated
– Destruction of repressor
cI
bio
gal
bio
gal
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
bio
gal
gal
gal
bio
gal
bio
bio
bio
Transduction
• Definition
• Types of transduction
• Significance
– Common in Gram+ bacteria
– Lysogenic (phage) conversion
• e.g. Corynebacterium diptheriae toxin
– Toxin derived from lysogenic phage
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)
Dono
r
– F (sex) pilus
– Recipient
• Lacks an F factor
Recipient
Physiological States of F Factor
• Autonomous (F+)
– Characteristics of F+ x Fcrosses
• 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- becomes F’
while F’ remains
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
F+
F-
F+
F-
– Origin of
transfer
– Rolling
circle
replication
F+
F+
F+
F+
Mechanism of Hfr x F- Crosses
• Pair formation
– Conjugation
bridge
• DNA transfer
Hfr
F-
Hfr
F-
– Origin of transfer
– Rolling circle
replication
• Homologous
recombination
Hfr
F-
Hfr
F-
Microbe Library, American Society for Microbiology
www.microbelibrary.org
Mechanism of F’ x F- Crosses
• Pair formation
– Conjugation
bridge
• DNA transfer
F’
F-
F’
F-
F’
F’
F’
F’
– Origin of transfer
– Rolling circle
replication
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
H1
flagella
IS
H2 gene
H2
flagella
Types of Transposable Genetic
Elements
• Transposons (Tn)
– Definition: Elements that carry other genes
in addition to those involved in
transposition
– Nomenclature - Tn10
Resistance Gene(s)
IS
– Structure
• Composite Tns
– Importance
• Antibiotic resistance
IS
Resistance Gene(s)
IS
IS
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)