bacterial genetics & bacteriophage pin lin ( 凌 斌 ), ph.d. departg ment of microbiology &...
TRANSCRIPT
Bacterial Genetics & Bacteriophage
• Pin Lin ( 凌 斌 ), Ph.D.
Departg ment of Microbiology & Immunology, NCKU
ext 5632
• References:
1. Chapters 5 in Medical Microbiology (Murray, P. R. et al; 5th edition)
2. 醫用微生物學 ( 王聖予 等編譯 , 4th edition)
Outline
• Introduction
• Replication of DNA
• Bacterial Transcription
• Other Genetic Regulation
(Mutation, Repair, &
Recombination)
Introduction
• DNA:
the genetic material• Gene:
a segment of DNA (or chromosome),
the fundamental unit of information in a cell• Genome:
the collection of genes• Chromosome:
the large DNA molecule associated with proteins or other components
Why we study Bacterial Genetics?
• Bacterial genetics is the foundation of the modern Genetic Engineering & Molecular Biology.
• The best way to conquer bacterial disease is to understand bacteria first.
Human vs Bacterial Chromosome
E Coli:
1. Single circular chromosome,
double-stranded; one copy (haploid)
2. Extrachromosomal genetic
elements:
Plasmids (autonomously self- replicating)
Bacteriophages (bacterial viruses)
3. Structurally maintained by, ex polyamines, spermine & spermidine
Human:
1. 23 chromosomes, two copies
(diploid)
2. Extrachromosomal genetic ele
ments:
- Mitochondrial DNA
- Virus genome
3. Maintained by histones
Replication of Bacterial DNA
1. Bacterial DNA is the storehouse of information.
=> It is essential to replicate DNA correctly and pass into the daughter cells.
2. Replication of bacterial genome requires several enzymes:
- Replication origin (oriC), a specific sequence in the
chromosome
- Helicase, unwind DNA at the origin
- Primase, synthesize primers to start the process
- DNA polymerase, synthesize a copy of DNA
- DNA ligase, link two DNA fragements
- Topoisomerase, relieve the torsional strain during the
process
Replication of Bacterial DNA
Features:
1.Semiconservative
2. Multiple growing forks
3. Bidirectional
4. Proofreading
(DNA polymerase)
Transcriptional Regulation in Bacteria
1. Bacteria regulate expression of a set of genes coordinately & quickly in response to environmental changes.
2. Operon: the organization of a set of genes in a biochemical pathway.
3. Transcription of the gene is regulated directly by RNA polymerase and “repressors” or “inducers” .
4. The Ribosome bind to the mRNA while it is being transcribed from the DNA.
Lactose Operon
1. E Coli can use either Glucose or other sugars (ex: lactose) as the source of carbon & energy.
2. In Glu-medium, the activity of the enzymes need to metabolize Lactose is very low.
3. Switching to the Lac-medium, the Lac-metabolizing enzymes become increased for this change .
4. These enzymes encoded by Lac operon:
Z gene => b-galactosidase => split disaccharide Lac into
monosaccharide Glu & Gal
Y gene => lactose permease => pumping Lac into the cell
A gene => Acetylase
Lactose Operon- Negative transcriptional regulation
Negative control
Repressor
Inducer
Operator
Lactose operon:
Lactose metabolism
Under positive or negative control
Positive control
Activator: CAP (catabolite gene-activator protein)
CAP RNA pol
Inducer
Lactose Operon- Positive Control
Tryptophan operon
Transcriptional Regulation (Example II)
-Tryptophan operonNegative control- Repressor- Corepressor (Tryptophan)- Operator
Attenuation
Transcription termination signal
Couple Translation w/ Transcription
Sequence 3:4 pair
-G-C rich stem loop
- Called attenuator
-Like transcriptional terminator
Sequence2: 3 pair
- weak loop won’t block translation
MutationTypes of mutations1. Base substitutions
Silent vs. neutral; missense vs. nonsense2. Deletions3. Insertions4. Rearrangements: duplication, inversion, transposition
May cause frameshift or null mutation
Induced mutationsPhysical mutagens:
e.g., UV irradiation (heat, ionizing radiation)
Chemical mutagens
Base analog
Frameshift
intercalating agents
Base modification
Transposable elements
Mutator strains
DNA Repair
1. Direct DNA repair
(e.g., photoreactivation)
2. Excision repair
Base excision repair
Nucleotide excision repair
3. Postreplication repair
4. SOS response: induce many genes
5. Error-prone repair: fill gaps with random sequences
Thymine-thymine dimer formed by UV radiation
Excision repair
Nucleotide excision repair
Base excision repair
Double-strand break repair(postreplication repair)
SOS repair in bacteria
1. Inducible system used only when error-free
mechanisms of repair cannot cope with
damage
2. Insert random nucleotides in place of the
damaged ones
3. Error-prone
Gene exchange in bacteriaMediated by plasmids and phages
PlasmidExtrachromosomal
Autonomously replicating
Circular or linear (rarely)
May encode drug resistance or toxins
Various copy numbers
Some are self-transmissible
Bacteriophage (bacterial virus)
Icosahedral tailess
Icosahedral tailed
Filamentous
Structure and genetic materials of phages
Coat (Capsid)
Nucleic acid
Lysogenic phaseLytic phase
Life cyclePhage as an example
Virulent phages: undergo only lytic cycle
Temperate phages: undergo both lytic and lysogenic cycles
Plaques: a hollow formed on a bacterial lawn resulting from infection of the bacterial cells by phages.
Mechanisms of gene transfer
Transformation: uptake of naked exogenous DNA by living cells.
Conjugation: mediated by self-transmissible plasmids.
Transduction: phage-mediated genetic recombination.
Transposons: DNA sequences that move within the same or between two DNA molecules
Importance of gene transfer to bacteria
• Gene transfer => a source of genetic variation => alters the genotype of bacteria.
• The new genetic information acquired allows the bacteria to adapt to changing environmental conditions through natural selection.
Drug resistance (R plasmids)
Pathogenicity (bacterial virulence)
• Transposons greatly expand the opportunity for gene movement.
Natural transformation
Transformation
Artificial transformation(conventional method and electroporation)
Demonstration of
transformation
Avery, MacLeod, and McCarty (1944)
Conjugationmediated by
self-transmissible plasmids
(e.g., F plasmid; R plasmids)
F’ plasmid
Hfr strain
F plasmid
F plasmid can integrate into bacterial chromosome to generate Hfr (high frequency of recombination) donors
Excision of F plasmid can produce a recombinant F plasmid (F’) which contains a fragment of bacterial chromosomal DNA
F plasmid
--an episome
Transductionphage-mediated genetic recombination
Generalized v.s. specialized transduction
TransposonsMobile genetic elements
May carry drug resistance genes
Sometimes insert into genes and inactivate them (insertional mutation)
Trans-Gram gene transfer
Spread of transposon throughout a bacterial population
Mechanisms of evolution of Vancomycin-resistant Staphylococcus Aureus
Cloning
Cloning vectors
plasmids
phages
Restriction enzymes
Ligase
In vitro phage packaging
Library construction
Genomic library
cDNA library
1. Construction of industrially important bacteria
2. Genetic engineering of plants and animals
3. Production of useful proteins (e.g. insulin, interferon,
etc.) in bacteria, yeasts, insect and mammalian cells
4. Recombinant vaccines (e.g. HBsAg)
Applications of genetic engineering