Unformatted text preview: MMG 461: Molecular Pathogenesis
Introduction to bacterial genetics and horizontal gene transfer January 14, 2007 H.A. Phelps Introduction to bacterial genetics
Review of important points about prokaryotic genetics: Bacteria are haploid Single circular chromosome No nuclear membrane Messenger RNA synthesis and translation can occur simultaneously in prokaryotes Translation mRNA 2 mRNA 3 Ribosome mRNA 1 Protein DNA initiation site RNA polymerase Transcription What is Horizontal Gene Transfer?
(lateral gene transfer) The transfer of genetic material from a donor bacterial cell to a recipient bacterial cell, even between different species A major driving force of prokaryotic evolution How can we detect cases of horizontal gene transfer? Difference in G+C content between donor and recipient sequence Regions adjacent to laterally transferred genes will contain remnants of translocatable elements Distribution of horizontally acquired DNA Ochman H, Lawrence JG, Groisman EA. Nature. 2000; 405: 299 304. Genetic Elements involved in horizontal gene transfer
1. Plasmids 2. Bacteriophage 3. Transposons 4. Intergrons 5. Genomic Islands Mechanisms of genetic transfer
1. Conjugation 2. Transduction 3. Transformation Plasmids Extrachromosomal doublestranded DNA molecules Encode all genes necessary for replication of the plasmid Often encode properties that are required for survival in specific environments 1. Antibiotic resistance genes = R plasmids 2. Heavy metal resistance 3. Growth on unusual substrates 4. Restriction/modification enzymes = protect DNA, degrade unprotected DNA 5. Bacteriocins = proteins/peptides toxic to other bacteria 6. Toxins = virulence plasmids Bacteriophage www.nsf.gov www.scienceclarified.com Insertion Sequence Elements (IS Elements) and Transposons www.uark.edu Insertion sequence elements are made up of a transposase gene with terminal inverted repeats IS elements involved in transfer of genomic islands IS elements can insert into genes Transposon insertion nitro.biosci.arizona.edu Transposons
1. Transposition moves DNA molecules to new locations within a cell, but CANNOT mediate transfer from one cell to another alone! 2. Transfer to a recipient cell requires a vehicle for lateral transfer plasmids phage 1. Can result in the acquisition of antibiotic resistance genes or virulence genes 2. Can alter host DNA by inserting into a gene, in turn inactivating that gene and downstream genes within an operon, OR bring in a new promoter and activating downstream genes 3. Transposition does not require sequence homology between host DNA and the transposable element Integons Gene expression elements that incorporate promoterless genes Made up of three elements: 1. an attachment site, 2. integrase, and 3. a promoter that drives expression of the incorporated sequence Mobility of integrons requires IS elements, transposons, or conjugative plasmids Integrons can carry antibiotic resistance genes and virulence genes (acquisition of virulence determinants by Vibrio cholerae ) Integration of promoterless genes occurs by site specific recombination Gene Capture and Expression by Integrons Ochman H, Lawrence JG, Groisman EA. Nature. 2000; 405: 299 304. Site Specific Recombination life.nctu.edu.tw Schematic model of a genomic island Foreign DNA block is linked to a tRNA gene and flanked by direct repeats (DR). G+C content differs from that of the core genome Multiple related functions can be transferred together Secretion system and effector genes Int = integrase gene abc, def and ghi = genes encoding specific functions IS = insertion sequence element Pathogenicity islands (PAIs) Carry one or more virulence factors, often multiple genes with integrated function Adherence factors, toxins, iron uptake, invasion, secretion systems Present in the genome of pathogenic bacteria but absent from nonpathogenic relatives Large segments of DNA (10200 kb), which means they can encode multiple gene products Distinct GC content and codon usage pattern Often associated with tRNA genes, probably indicate phagederived elements In General : Conjugation Transfer of DNA from one cell to another through DIRECT physical contact between a donor cell and recipient cell Requires a special apparatus called the pilus for binding the donor and recipient cells, bring them into direct cell to cell contact DNA from the donor cell is transferred into the recipient cell through a membrane pore Double stranded plasmid DNA separates and one strand moves from the donor bacterium into the recipient bacterium; the two single strands serve as templates for replication of complete double stranded DNA molecules in both cells Transfer occurs by: 1. Selftransmissible or moblized plasmid 2. Promiscuous plasmid 3. Conjugative transposons 4. Can mediate transfer of chromosomal sequences Conjugation
Self transmissible plasmids trc.ucdavis.edu Conjugation
Conjugative transposons www.life.uiuc.edu Abigail Salyers Conjugation
Transfer of chromosomal sequence fig.cox.miami.edu Transduction www.nsf.gov www.scienceclarified.com Transduction Transfer of bacterial DNA from one cell to another by bacteriophage (phage) There are two types of transduction: be 1. General transduction any region of the bacterial DNA can transferred 2. Specialized transduction only those genes close to the attachment site of a lysogenic phage can be transferred Amount of DNA packaged is dependent of phage capsid size Phages are abundant in the environment Phage have specific host range Transduction fig.cox.miami.edu Generalized Transduction www.cbs.dtu.dk Specialized Transduction faculty.ircc.edu Transformation Naturally competent bacteria have the ability to uptake naked DNA from the environment Neisseria gonorrhoeae and Haemophilus influenzae are perpetually competent to accept DNA, but requires specific recognition sites Bacillus subtilis and Streptococcus pneumoniae become competent upon reaching a certain phase of growth, but do not display sequence preference Donor and recipient do not have to physically contact each other Potential to transmit DNA between distantly related organisms www.bio.davidson.edu Homologous Recombination Loss of genes also impacts virulence Ochman H, Lawrence JG, Groisman EA. Nature. 2000; 405: 299 304. Summary Frost LS, Leplae R, Summers AO, Toussaint A. Nat Rev Microbiol. 2005; 3: 72232. ...
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