The Genetics of Bacteria and Their Viruses 8 The Genetics of Bacteria and Their Viruses

Plasmids Plasmids are circular DNA molecules which replicate independently of the bacterial chromosome Plasmids often carry antibiotic resistance genes transferred to recipient cells by transformation Plasmids are used in genetic engineering as gene transfer vectors

Conjugation Conjugation = process in which DNA is transferred from bacterial donor cell (F+) to a recipient cell (F-) by cell-to-cell contact F (fertility) factor = plasmid transferred by conjugation F factor = episome = genetic element that can insert into chromosome or replicate as circular plasmid

Transposable Elements Transposable elements = DNA sequences present in multiple copies which are capable of movement within the genome Insertion (IS) elements = mobile elements contain transposase = enzyme catalyzes IS element transfer Transposons = IS elements which contain bacterial genes

Transposable Elements Transposons can insert into plasmids which can be transferred to recipient cells by conjugation Transposable elements are flanked by inverted repeats and often contain multiple antibiotic resistance genes= R plasmids

Bacterial Genetics Types of bacterial mutants: Antibiotic-resistant mutants Nutritional mutants: wildtype=prototroph mutant=auxotroph which cannot grow in minimal media providing basic nutrients only carbon-source mutants=cannot use some carbon sources

Bacterial Transformation Recipient cells acquire genes from DNA outside the cell DNA is taken up by cell and often recombines with genes on bacterial chromosome Bacterial transformation showed that DNA is the genetic material Transformation may alter phenotype of recipient cells

Cotransformation of Linked Genes Donor DNA which contains genes located close together are often transferred as a unit to recipient cell = cotransformation The greater the distance between genes the less likely they will be transferred as a unit to recipient cell Cotransformation is used to map gene order

Hfr Hfr (high frequency recombination) exchange between donor cells F+ and few cells where F factor integrated into bacterial chromosomes Recombination inserts Hfr genes into chromosome

Chromosome Mapping Time-of-Entry mapping = method of mapping genes by Hfr X F- matings using interrupted mating technique A plot of time (minutes) versus # of recombinants is used to map genes as transfer order map Circular genetic map of E. coli shows map distances of genes in minutes

Transduction Transduction = bacterial DNA fragment is transferred from one bacterial cell to another by a virus (phage) containing bacterial DNA = transducing phage Generalized transducing phage = transfers DNA derived from any part of the bacterial chromosome

Transduction Phage P1 cuts bacterial chromosome into pieces and can package bacterial DNA into phage particles Transducing particle will insert ‘transduced” bacterial genes into recipient cell by infection Transduced genes may be inserted into recipient chromosome by homologous recombination

Transduction Specialized transducing phage = particles contain phage and bacterial genes from a specific point of bacterial chromosome Cotransduction can be used to demonstrate linkage between bacterial genes Frequency of cotransduction is a measurement of linkage

Transduction Specialized transducing phages transduce bacterial genes at the site of prophage insertion into the bacterial chromosome Transduction of bacterial genes occurs by aberrant excision of viral DNA which results in the incorporation of bacterial genes into phage chromosome

Temperate Bacteriophages Temperate bacteriophages have two life cycles: lytic cycle=infection which results in production of progeny phage and bacterial cell lysis and lysogeny = non-productive viral infection results in insertion of viral DNA into bacterial chromosome Viral DNA integration= site-specific insertion into bacterial chromosome

Lysogenic Bacteriohages In the lysogenic cycle, the viral DNA integrated into the bacterial chromosome is called a prophage Lysogen=bacterial cell containing integrated prophage Integration is catalyzed by a viral enzyme= integrase which carries out site- specific recombination between the virus and bacterial cell

Lysogenic Bacteria Prophage induction= excision of prophage from bacterial chromosome and entry to lytic cycle Prophage induction results from damage to the bacterial chromosome by chemicals or radiation Excisionase=viral enzyme which removes prophage by site-specific recombination