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Genetic Variability in Bacteria
Mutationspontaneous vs inducible point mutations
silent, missense, nonsense, frameshiftforward vs reverse (reversion & suppression)
macrolesions
Genetic ExchangeTransformation
ConjugationTransduction
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.
Transduction• 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.
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 abcdefg → abfg – inserted abcdefg → abcxyzefg– Inverted abcdefg → abedcfg– duplicated. abcdefg → abcdecdefg
• Spread beneficial trait:– Intraspecific– Interspecific
• Antimicrobial resistance