Genetic Variability in Bacteria
Mutation
spontaneous vs inducible point mutations
silent, missense, nonsense, frameshift
forward vs reverse (reversion & suppression)
macrolesions
Genetic Exchange
Transformation
Conjugation
Transduction
Transposable elements
Genetic Exchange
• Recombination:
– Reciprocal: 2 x dsDNA with
common sequence regions swap
(cross-over)
– Nonreciprocal: ssDNA forms
heteroduplex with dsDNA
– Site-specific
– Transposition
•
•
•
•
Transformation
Conjugation
Transduction
Transposons
See Holliday Model (Fig. 13.2 )
Griffith’s Transformation (1928)
Avery, MacLoed & McCarty (1944) concluded DNA!
Transformation
• DNA fragments from
environment get
recombined into the
chromosome.
• Natural mechanisms of
DNA uptake vary
among bacteria.
• Artificially facilitated by
chemically treating
cells (competent cells),
or by electrical pulse.
Conjugation (“Bacterial Sex”)
•
•
•
•
•
F+ cell is donor.
F- cell is recipient.
F+ x F- mating.
Bound by F+ cell pilus
Transfer by rolling circle
mechanism.
F-Plasmid
Replication
for Transfer
• “Rolling Circle
Mechanism”
–
–
–
–
Relaxosome nicks DNA
Extend at 3’-OH.
Displace parent strand.
Synthesize complement for
displaced strand.
• Viral Replication
– Same mechanism
– Multiple copies made
from a single initiation.
F-factor Integration
• F-factor recombines (single crossover) with F+
cell chromosome at an insertion sequence (IS).
• High frequency of recombination Hfr cell created.
• Precise de-integration reverses Hfr back to F+.
• Imprecise de-integration converts Hfr to F’.
IS
IS Facilitated Plasmid Integration
What is an F’?
Hfr
F’
Chromosomal Gene Exchange
• Hfr or F’ cells may result in transfer and
recombination of chromosomal genes to F- cell.
• F’ factor has chromosomal DNA; transfers like a
normal F factor to the recipient, making a new F’.
• Hfr can initiate transfer via the rolling circle
mechanism; typically transfer of the chromosome
is incomplete; only a fragment gets incorporated.
• Viruses of bacteria are
called a phage.
• Phage protein coat
surrounding DNA is called a
capsid.
• During phage replication
and assembly, capsids may
package chromosomal or
plasmid DNA by mistake.
• When transferred to a new
host it may recombine.
Transduction
Transducing particle
Transposable Elements:
“Jumping Genes”
• Transposable elements (insertion sequences and transposons) can tranfer copies
of themselves within or to other DNA molecules (chromosome, pDNA, or vDNA).
• Antibiotic resistance genes rapidly spread within and between bacterial populations
by composite transposons carried on F factors called R plasmids.
Self-replicative
recombination
•Transposon or IS self-replicates
copy to splice into DNA at a
specific target sequences.
• Endonuclease activity cuts
target sequence, leaving single
strand overhanging ends.
•Transposon is ligated to ends.
• Gaps are filled by DNA
polymerase to yield a target
sequence at each side of the
transposon (called direct
repeats).
• Specific details of the process
are more involved (Fig 13.10).
Effects of Transposon Activity
• Insert to mutate genes; change phenotype.
• Influence transcription or translation (on/off)
– Promoters activate
– Stop codons and terminator sequences inactivate
– Useful tool in gene purification and function studies
• Facilitate plasmid fusion.
• Conjugative transposons with transfer genes.
• Transfer antibiotic resistance.
– Plasmid to plasmid
– Plasmid to chromosome
Consequences of
Genetic Exchanges
• Facilitate macrolesions; mutations of large
sequence regions of DNA:
– deleted
– inserted
– Inverted
– duplicated.
abcdefg →
abcdefg →
abcdefg →
abcdefg →
• Spread beneficial trait:
– Intraspecific
– Interspecific
• Antimicrobial resistance
abfg
abcxyzefg
abedcfg
abcdecdefg