Chapter 9Genetics of Bacteria and Their Viruses:
Transduction and phage genetics
Jones and Bartlett Publishers © 2005
Transduction
• Lytic cycle – the phage attaches to a bacterium then injects its nucleic acid. More copies of the phage are made by the infected bacterium, then the cell lyses to release the new phage particles.
• Phage that only have a lytic cycle are called virulent.
• Phage research has helped scientists understand prokaryotic genetics.
• Phage are 1/100 to 1/500th the size of a bacterium.
• They can be lytic or lysogenic.
• Lytic = virulent.
• Lysogenic = prophage.
• Temperate phage = do both.
Life cycle of a bacteriophage
Phage structure
Life cycle of bacteriophage T4
Assembly of bacteriophage T4
Life cycle of theLambda phage
Generalized transduction of bacterial genes mediated by bacteriophage P1
When a bacteriophage can
transfer any bacterial gene, the process is called generalized
transduction
Transduction can be used for mapping genes at a higher resolution than is possible by conjugation mapping
Bacteriophage genetics
• Phage progeny generally are identical to their parent (except for mutations).
• If two or more phage particles infect a single bacterial cell simultaneously, new phage genotypes can arise.
• This is different from eukaryotic recombination in two ways:
• The number of participating DNA molecules differs from cell to cell.
• Reciprocal recombinants are not always recovered in equal frequencies.
Infection of a bacterial lawn by a lytic bacteriophage creates a clear spot (plaque) due to the lysis of bacteria
Results of a number of phage crosses can be combined to generate the complete genetic map of a
bacteriophage chromosome
The genetic map of bacteriophage T4
Bacteriophage T4 chromosome is approximately
167,000 base pairs in size. The
chromosome is linear but the genetic map is
circular
Bacteriophage T4 chromosome is circularly permuted and terminally redundant
Mapping of bacteriophage T4 deletion mutants in rapid lysis genes (rII mutants) using overlapping deletions
The logic of mapping of rII deletion mutants by crossing to a set of ordered overlapping
deletion mutants
A high resolution genetic map of the rIIA and rIIB genes showing 2 hot spots of mutation
Each small square in this map represents an independently isolated mutation. A very large number of mutations
occur at 2 sites, one in rIIA gene and one in
rIIB gene.
A genetic map of the bacteriophage showing the genes and the transcripts
Insertion of bacteriophage into the bacterial chromosome and silencing of the
lytic genes creates a lysogenic cell
Upon infection, the sticky ends of the bacteriophage
DNA join to make a circular molecule
Cleavage sites and reciprocal recombination
Overview of integration into the bacterial chromosome
Site-specific recombination is involved in the insertion of bacteriophage DNA to create a lysogen
Maps of the lytic virus and the prophageare circular permutations of each other
Abnormal excision of the prophage transfers bacterial genes to the excised phage and
leaves behind phage genes
The “d” in the name of the
excised phages (dgal or dbio)
stands for defective. Phage genes were lost during the pick up of the host
genes.