TRANSFERIMIENTO LATERAL DE GENES

Fig. 8.7 transmission electron micrograph of F pilli of E. coli Unlike the shorter attachment pili (fimbriae), this long type of pilus is used for transfer of genes in conjugation and is often called a sex pilus. F+ F- Fig. 8.7 transmission electron micrograph of F pilli of E. coli

F' cells Formation of an F' cell from an Hfr cell, and transfer of a bacterial chromosome segment to a recipient cell.

F+ F- An F+ x F- mating- The F+ cell transfers one strand of DNA from its plasmid to the F- cell via the conjugation bridge. As this occurs, the complementary strands of F plasmid DNA are synthesized. Fig. 8.8 An F+ x F- mating.

F', formed by the excision of the F factor plasmid from the Hfr strain contains some host genes in addition to the gene for F plasmid. The F' transfers its genetic material like an F+ strain Fig. 8.10 The formation and transfer of F’ plasmids

Viral reproduction: the lytic cycle Generalized schematic for viral reproduction in a host bacterium, through the lytic cycle. In the lytic cycle, the virus (phage) multiplies in the host cell and the progeny viruses are released by lysis of cell.

Viral reproduction: the lysogenic cycle Generalized schematic for viral reproduction in a host bacterium, through the lysogenic cycle. In the lysogenic cycle, viral DNA is integrated into the host genome and replicates as the chromosome replicates, producing lysogenic progeny cells

Generalized transduction: Lytic phage

Specialized transduction: Lysogenic phage

Transposable elements in prokaryotes Insertion sequence (IS) elements Transposons (Tn) Bacteriophage Mu

Insertion elements and transposons Insertion sequences (IS) are short DNA sequences, about 700 to 5000 bp which can move from one location in a DNA sequence to another. They have short 16-41 bp inverted repeats on their ends. They encode a transposase which catalyses site-specific recombination. Simple transposons are mobile genetic elements in which a one or more genes are flanked by two insertion sequences.

Composite transposons Structures of some bacterial transposable elements. A composite transposon contains antibiotic genes flanked by two insertion sequences as direct or inverted repeats Shown here is the Tn5 transposon, with inverted repeats. The Tn3 transposon.

Insertion sequence (IS) elements Simplest type of transposable element found in bacterial chromosomes and plasmids Encode only genes for mobilization and insertion Range in size from 768 bp to 5 kb IS1 first identified in E. coli’s glactose operon is 768 bp long and is present with 4-19 copies in the E. coli chromosome Ends of all known IS elements show inverted terminal repeats (ITRs)

Integration of IS element in chromosomal DNA

Three different mechanisms for transposition Conservative transposition Replicative transposition Retrotransposition

Three different mechanisms for transposition Conservative transposition: The element itself moves from the donor site into the target site Replicative transposition: The element moves a copy of itself to a new site via a DNA intermediate Retrotransposition: The element makes an RNA copy of itself which is reversed-transcribed into a DNA copy which is then inserted (cDNA)

Conservative transposition

Replicative transposition

Retrotransposition

common feature of mobile elements Generation of short direct repeats flanking the newly inserted element This results for a staggered cut being made in the DNA strands at the site of insertion

Transposons (Tn) Similar to IS elements but are more complex structurally and carry additional genes 2 types of transposons: Composite transposons Noncomposite transposons

Composite transposons

IS10R is an autonomous element, while IS10L is non-autonomous

Composite Transposons Tetracycline resistance is carried by a transposable element The transposon is a composite transposon, composed of IS-elements flanking an included sequence, in this case containing an antibiotic resistance gene IS10R is an autonomous element while IS10L is non-autonomous Composite transposons probably evolved from IS elements by the chance location of a pair in close proximity to one another. Inactivation of one element by mutation would not harm ability to transpose and would assure continued transposition of the entire transposon

Noncomposite transposons

Noncomposite transposons (Tn) Carry genes (e.g., a gene for antibiotic resistance) Ends are non-IS element repeated sequences Tn3 is 5 kb with 38-bp ITRs and includes 3 genes; bla (-lactamase), tnpA (transposase), and tnpB (resolvase, which functions in recombination)

Examples of DNA-intermediate mobile elements Insertion Sequences (IS) elements in bacteria P elements in Drosophila AC/DS (dissociation) elements in maize AC is a full-length autonomous copy DS is a truncated copy of AC that is non-autonomous, requiring AC in order to transpose At least seven major classes of DNA transposons in the human genome (3% of total genome)

Structure and transposition of a transposable element