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