Genetic Transfer in Bacteria: Transduction Mechanism

Genetic Transfer in Bacteria

• There are three mechanisms of genetic transfer in bacteria: transformation, transduction, and conjugation.

Transduction

• A bacteriophage infects a donor cell and incorporates a fragment of bacterial chromosomal DNA into a newly made phage particle.
• The phage then transfers this fragment of DNA to a recipient cell, which incorporates the DNA into its chromosome by recombination.

Transformation

• When a bacterial cell dies, it releases a fragment of its DNA into the environment.
• This DNA fragment is taken up by a recipient cell, which incorporates the DNA into its chromosome by recombination.

Conjugation

• Genetic transfer in bacteria was discovered in 1946 by Joshua Lederberg and Edward Tatum.
• They were studying strains of Escherichia coli that had different nutritional growth requirements.
• Auxotrophs cannot synthesize a particular nutrient and therefore need it in their growth medium.
• Prototrophs make all their nutrients from components in their growth medium.

Lederberg and Tatum's Experiment

• One strain of E. coli was designated met bio thr leu thi, requiring one vitamin (biotin) and one amino acid (methionine).
• Another strain was designated met+ bio+ thr- leu- thi-, requiring one vitamin (thiamine) and two amino acids, leucine and threonine.
• The genotype of the bacterial cells that grew on the plates must be met bio thr leu thi.
• Lederberg and Tatum reasoned that some genetic material was transferred between the two strains.

Conjugation Requires Direct Physical Contact

• Bernard Davis later showed that the bacterial strains must make physical contact for transfer of genetic material to occur.
• Davis placed the two strains on opposite sides of a filter in a U-tube apparatus.
• Application of pressure or suction promoted the movement of liquid through the filter.
• Samples of bacteria from either side were then placed on growth media that selected for the met+ bio+ thr+ leu+ thi+ genotype.
• Nothing grew! Davis concluded that the bacteria must make direct physical contact to transfer genetic material.

F Factors and Conjugation

• In E. coli, certain donor strains contain a small circular piece of DNA termed an F factor (for fertility factor).
• Strains containing an F factor are designated F+, while those lacking it are F-.
• Genes that play a role in the transfer of DNA are named tra or trb followed by a capital letter.
• Contact initiates conjugation, which is mediated by sex pili (or F pili) made only by F+ strains.

Mechanism of Transfer

• The first step in conjugation is the contact between donor and recipient cells.
• This is mediated by sex pili, which act as attachment sites for the F- bacteria.
• Once contact is made, the pili shorten, and the donor and recipient cells are drawn closer together, forming a conjugation bridge.
• The successful contact stimulates the donor cell to begin the transfer process.
• The mechanism of transfer involves the relaxosome, relaxase, and an exporter complex of proteins that spans both inner and outer membranes.

Plasmids

• A plasmid is the general term used to describe extra-chromosomal DNA.
• Most are circular, although some are linear.
• Plasmids are present in many bacteria and a few eukaryotic species.
• They range in size from a few thousand to 500,000 bp.
• Some plasmids, called episomes, can integrate into a chromosome.
• Plasmids have their own replication origin and can replicate independent of the bacterial chromosome.

Five Different Categories of Plasmids

• Fertility plasmids allow conjugation.
• Resistance plasmids (R factors) contain genes conferring resistance to antibiotics.
• Degradative plasmids carry genes allowing digestion of unusual substances.
• Col-plasmids contain genes that kill other bacteria.
• Virulence plasmids carry genes that turn bacteria into pathogenic strains.