Chapter 9 Genetics 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 the Lambda 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 l into the bacterial chromosome and silencing of the lytic genes creates a lysogenic cell

Upon infection, the sticky ends of the bacteriophage l DNA join to make a circular molecule

Cleavage sites and reciprocal recombination

Overview of l integration into the bacterial chromosome

Site-specific recombination is involved in the insertion of bacteriophage l DNA to create a lysogen

Maps of the lytic virus and the prophage are circular permutations of each other

Abnormal excision of the l prophage transfers bacterial genes to the excised phage and leaves behind phage genes The “d” in the name of the excised phages (ldgal or ldbio) stands for defective. Phage genes were lost during the pick up of the host genes.