Exchange of Genetic Information
Dr. Jeffrey Patton
Associate Professor
Pathology, Microbiology, and Immunology
USC-School of Medicine
TEACHING OBJECTIVE S:
1. To explain the mechanisms of gene transfer in
bacteria.
2. To describe the nature of transposable g enetic
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)
