Lateral Transfer

Donating Genes

• Mutation often disrupts the function of a gene

• Gene transfer is a way to give new functions to the recipient cell

• Thus, gene transfer is a quicker way for a population to generate versatility and diversity

Gene Transfer

• Can involve the transfer of an entire plasmid

• Can also be parts of chromosomes

• Recombination is needed to place the parts into a stable chromosome

• Recombination is the exchange of genes (or parts of genes) between two DNA molecules (or parts of the same molecule)

Lateral Transfer

• Antibiotic resistance• Virulence factors• Metabolic enzymes, i.e. Pseudomonas

species have plasmids for degradation of petroleum hydrocarbons

• Other non-essential (but very useful) functions

• The DNA that is swapped is often a plasmid

Plasmids that have made the rounds

• R100 – genes for resistance to sulfonamides, streptomycin, tetracycline, chloramphenicol, mercury

found in Escherichia, Klebsiella, Salmonella (enterics)

• Penicillinase-producing plasmid in Neisseria may be from Streptococcus

• E.coli O157:H7 has a Shigella plasmid

(mercury resistance)

(sulfonamide res.)

(streptomycin res.)

(chloramphenicol res.)

(tetracycline res.)See also fig. 8.28b

Transformation

• Naked DNA is transferred from one cell to another

• In nature, it often follows the death and lysis of one cell

• Transformation must occur before the DNA is completely degraded

Transformation

• A whole plasmid or chromosomal fragments can be transferred

• The fragments must be integrated by recombination

• Stable transformation the new DNA is inherited by the progeny

Fig. 8.24

Competence

• The DNA must pass through the cell wall and cell membrane

• Not all bacteria are naturally competent – some Neisseria, Streptococcus, Staphylococcus are

• They can be made so in the lab with Ca++ or with electric shock

Some History

• 1928 – Frederick Griffith works with S. pneumoniae

• One strain has a capsule and is virulent

• Another strain has no capsule and does not cause disease

• The virulence of the first strain can be eliminated by heat-killing the cells

Fig. 8.23

Griffith’s Experiments

• Injecting heat-killed encapsulated strain along with the avirulent strain causes disease

• He even isolated live, encapsulated S. pneumoniae from the diseased mice

• The avirulent strain was transformed by some genetic material from the virulent strain (Griffith did not know that is was DNA)

Conjugation

• Requires that cells make direct contact

• The two cells are often of opposite mating type

• Can be interspecies

• Gram-negative produces sex pili

• Gram-positives use other surface molecules for attachment

Fig. 8.25

Conjugation

• A plasmid is transferred from an F+ cell to an F- cell

• One stand of the plasmid is transferred

• Replication occurs in each cell

F+X

See also fig. 8.26a

Fig. 8.26b & c

Agrobacterium tumefaciens

• Transfers genes to plants!

• Lateral transfer is not limited to prokaryotes

• Can eukaryotes transfer their DNA? Unknown, except by transformation

Crown gall tumor

S.B. Gelvin (2005) Nature 433: 583-4.

Recent Studies

• “Transfer of carbohydrate-active enzymes from marine bacteria to Japanese gut microbiota.” Nature. 4/8/10.

• “The shared antibiotic resistome of soil bacteria and human pathogens.” Science. 8/31/12.

Transduction

• The DNA is carried by an intermediary

• The intermediary is a bacteriophage (virus)

• Normally, the phage carries its own DNA

• Occasionally, it randomly picks up some bacterial DNA during infection

Transduction

• Phage injects its DNA into host cell

• Cell replicates phage DNA and translates phage proteins

• Cellular DNA is fragmented

• Some phage incorporated pieces of the bacterial DNA

• In some cases, the next round of infection will involve transfer of bacterial DNA

Fig. 8.27

Transduction

• Often mediated by a prophage during the lysogenic phase of infection

• During lysogeny, the phage DNA is recombined into the host chromosome for any # of generations (the phage DNA is the prophage)

Transduction

Phage Conversion

• Some pathogens rely on prophage DNA for their virulence

• Corynebacterium diptheriae

• Clostridium botulinum

• Vibrio cholerae

• Streptococcus pyogenes

Natural Selection

• One of the keys to natural selection is the existence of a diverse or versatile population

• Lateral transfer of genes is a potent way to generate versatility (more so than mutation)

• Lateral transfer takes full advantage of already occurring diversity

• Remember, this transfer often occurs across species

Transposons – Mobile DNA

• Originally termed “jumping genes” by Barbara McClintock in the 1950s

• Transposons are DNA sequences

• They can move from one place to another – on the same chromosome, from chromosome to chromosome, between plasmid and chromosome, into prophage DNA, etc.

Transposons

• Always contain insertion sequences and a gene encoding a transposase enzyme

• Transposase opens a piece of DNA and seals it around the transposon (which gets inserted into the break)

• Transposons can also contain additional genes, i.e. those for antibiotic-resistance

• The transposon may disrupt other genes when jumping

Frequency of 10-5 to 10-7 per generation Red = insertion sequencestnp = transposase gene

Transposon

kan = kanamycin-resistance genestr = streptomycin-resistance genebleo = bleomycin-resistance gene

Transposition

One copy can become mutated in future generations

See also fig. 8.29