Molecular Genetics

Cotransduction of genetic markers in bacteria can be used in genetic map making.

Facts

	Measurement of conjugal transfer of markers is one way to map bacterial chromosomes. When bacteriophages infect cells, proteins are produced that package replicated phage genomes into new phage particles. In the example at left the lysogenic P1 phage resident in a leu+ bacterium is induced. The resulting phage are used to infect leu- bacteria. The bacteria are plated on media lacking leucine, thus selecting for complementation of the defect in the leucine biosynthetic pathway. In the experiment diagrammed to the right, the recipient bacteria contain mutations in genes at the extreme ends of a conjugation map. The frequency of transducing both genes together is not much less than that of transducing adjacent genes.

Interpretations-left

• Packaging of genomes is imprecise. Some fragments of the bacterial chromosome are packaged at low frequency.
• When these fragments contain the leu gene, the recipient bacteria are transformed to leucine independence.
• The process is called transduction and P1 is a transducing phage.

Interpretations-right

• The transduction of two genes by one phage into the same recipient cell is called cotransduction.
• Cotransduction frequencies can be used for fine mapping of genes.
• The cotransduction of markers on opposite ends of the map derived from conjugation is part of the evidence that most bacterial chromosomes are actually circular.
• The use of conjugating strains with different origins of transfer results in lineraly permuted maps, also implying that the E.coli chromosome is circular.

Further information

• Genetic mapping requires traits whose behavior can be analyzed.