Bacterial Genetics G.Jamjoom 2005
Dec 19, 2015
Bacterial Genetics
Lecture Outline :
1. The study of bacterial genetics helped illustrate: - the nature of genetic material as DNA - the genetic code - the nature of mutations (changes in nucleotide sequences) - regulation of gene function (repressors, activators)
DNA Forms the Genetic Information:
- The Griffith experiment (1928,1944):
DNA fragments from capsulated pneumococcus can give noncapsulated strain the ability to make capsule -
“ transformation”
DNA Forms the Genetic Information:
- The Hershey and Chase experiment
Bacteriophage DNA alone enters the bacterial cell and makes new progeny phages. Phage protein coat remains outside and is not involved in this process.
Length of DNA In Different Organisms
- Bacteriophage MS2 Virus 4,000 bp ~10 genes
- Bacteriophage T2 Virus 21,000 bp ~ 200 genes
- Escherichia coli Bacterium 4,000,000 bp ~4288 genes
- Saccharomyces Yeast 14,000,000 bp
- C. elegans Nematode 100,000,000 bp
- A. thaliana plant 100,000,000 bp - Drosophila Insect 165,000,000 bp - Mouse Mammal 3,000,000,000 bp - Human Mammal 3,500,000,000 bp ~ 40,000genes
ProkaryotesProkaryotes
• EubacterEubacter "True" bacteria "True" bacteria – human pathogenshuman pathogens
– clinical or environmentalclinical or environmental
– one kingdomone kingdom
• ArchaeaArchaea – Environmental organisms Environmental organisms
– second kingdom second kingdom
Bacterial Genetics
Lecture Outline:
3. The bacterial chromosome :
- structure, genes, operons
- mapping
- complete sequences of selected bacteria
- replication, transcription, translation
The Bacterial Chromosome
• Most bacterial chromosomes are circular
• Many have been fully sequenced
• Many genes have been identified and mapped using gene transfer techniques such as conjugation, transduction, and transformation
The Complete Sequence of Escherichia coli Chromosome
Echerichia coli chromosome
• Size 4,600,000 base pairs (4.6 megabases)• Contains 4288 genes ( 62% identified)• Many genes code for the following : - Cell structure - Energy metabolism - Proteins form DNA replication - Proteins for transcription, translation, RNA synthesis - Synthesis of amino acids , nucleotides, etc. - Synthesis of enzymes• Contains transposons and plasmid and phage
sequences
Bacterial Genetics
Lecture Outline:
6. Bacteriophages (bacterial viruses):
- virulent
- temparate:
lysogey
G.Jamjoom 2005
Phage Composition and Structure
Tail
Tail Fibers
Base Plate
Head/Capsid
Contractile Sheath
Genomic DNA
Types of Bacteriophage
Lytic or virulent: Phage that multiply within the host cell ,
lyse the cell and release progeny phage (e.g. T4) Lysogenic or temperate phage: Phage that can either multiply via the lytic cycle
or enter a quiescent state in the bacterial cell. (e.g., )– Expression of most phage genes repressed– Prophage – Phage DNA in the quiescent state– Lysogen – Bacteria harboring a prophage
Bacterial Genetics
Lecture Outline:
4. Plasmids (extrachromosomal elements):
- functions
- role in antibiotic resistance (R plasmids)
Plasmids
• Definition: Extrachromosomal genetic elements that are capable of autonomous replication (replicon)
• Episome - a plasmid that can integrate into the chromosome
Plasmid- Coded FunctionsPlasmid- Coded Functions
• Fertility • Resistance to: - antibiotics - irradiation - phages• Production of : - exotoxins - enterotoxins - bacteriocins - Proteases (cheese)
• Metabolism of : - various sugars - hydrocarbons• Tumergenesis in
plants
Bacterial Genetics
Lecture Outline:
7. Mechanism of gene transfer in bacteria:
- Transformation
- Transduction
- Conjugation
Transduction
• Types of transduction– Generalized Transduction : in which potentially
any dornor bacterial gene can be transferred.
– Specialized Transduction : in which only
certain donor genes can be transferred
Generalized Transduction
• Release of phage
• Phage replication and degradation of host DNA
• Assembly of phages particles
• Infection of recipient• Homologous recombination
• Infection of Donor
Potentially any donor gene can be transferred
Events Leading to Lysogeny
• Site-specific recombination
–Phage coded enzyme
• Repression of the phage genome
– Repressor protein– Specific– Immunity to superinfection
gal bio
gal bio
gal
bio
Termination of Lysogeny• Induction
– Adverse conditions
• Role of proteases– recA protein– Destruction of
repressor
• Excision• Lytic growth
gal
bio
gal bio
gal bio
gal bio
• Gene expression
Specialized TransductionLysogenic Phage
• Excision of the prophage
gal
bio
gal bio
gal bio
gal
bio
bio
gal
• Replication and release of phage
• Infection of the recipient
• Lysogenization of the recipient– Homologous
recombination also possible
Conjugation
• Definition: Gene transfer from a donor to a recipient by direct physical contact between cells
• Mating types in bacteria– Donor
• F factor (Fertility factor)– F (sex) pilus
Donor
Recipient
– Recipient• Lacks an F factor
Physiological States of F Factor
• Autonomous (F+)–Characteristics of F+ x F-
crosses• F- becomes F+ while F+ remains F+
• Low transfer of donor chromosomal genes
F+
Physiological States of F Factor
• Integrated (Hfr)– Characteristics of
Hfr x F- crosses• F- rarely becomes
Hfr while Hfr remains Hfr
• High transfer of certain donor chromosomal genes
F+ Hfr
Physiological States of F Factor
• Autonomous with donor genes (F’)– Characteristics of F’
x F- crosses
• F- becomes F’ while F’ remains F’
• High transfer of donor genes on F’ and low transfer of other donor chromosomal genes
Hfr F’
Mechanism of F+ x F- Crosses
• DNA transfer– Origin of
transfer– Rolling circle
replication
• Pair formation
– Conjugation bridge
F+ F- F+ F-
F+ F+F+ F+
Mechanism of Hfr x F- Crosses
• DNA transfer– Origin of transfer– Rolling circle
replication
• Homologous recombination
• Pair formation
– Conjugation bridge
Hfr F- Hfr F-
Hfr F-Hfr F-
Mechanism of F’ x F- Crosses
• DNA transfer– Origin of transfer– Rolling circle
replication
• Pair formation
– Conjugation bridge
F’ F’F’ F’
F’ F- F’ F-
Conjugation
• Significance– Gram - bacteria
• Antibiotic resistance• Rapid spread
– Gram + bacteria• Production of adhesive material by donor cells
Bacterial Genetics
Lecture Outline:
5. Transposons (jumping genes) :
- role in antibiotic resistance
G.Jamjoom 2005
Transposons(Transposable Genetic Elements)
• Definition: Segments of DNA that are able to move from one location to another
• Properties– Inverted terminal repeat sequences (loop formation)– “Random” movement from one DNA site to another– Not capable of self replication (not a replicon)– Transposition mediated by site-specific recombination
• Transposase– Transposition may be accompanied by duplication
Examples of Antibiotic Resistance Transposons
• Tn 1 ampicillin• Tn5 kanamycin• Tn6 Trimethoprim• Tn9 Chloramphenicol• Tn10 Tetracyclin• Tn551 erythromycin
Structure of R Factors
• RTF– Conjugative
plasmid– Transfer genes
Tn 9
Tn
21
Tn 10
Tn 8
RTF
R determinant
• R determinant– Resistance genes– Transposons
Mechanism of Plasmid-Mediated Resistance
• Production of enzymes for : - Hydrolysis of β-lactam ring - phosphorylation - adenylation - acetylation - methylation - modification of permeability - other
Control of Gene Expression
• Transcriptional control
• Clustering of genes with related function
• Coordinate control of genes with related function
• Polycistronic mRNA
Inducible Genes - Operon Model
• Definition: Genes whose expression is turned on by the presence of some substance– Lactose induces expression of the lac genes– An antibiotic induces the expression of a
resistance gene
Lactose Operon
• Structural genes– lac z, lac y, & lac a– Promoter– Polycistronic
mRNA
• Regulatory gene– Repressor
• Operator• Operon• Inducer - lactose
i
Operon
RegulatoryGene
p o z y a DNA
m-RNA
-GalactosidasePermease
Transacetylase
Protein
Lactose Operon
• Inducer -- lactose– Absence
• Active repressor• No expression
i p o z y a
No lac mRNA
Absence of lactose
Active
i p o z y a
-Galactosidase Permease Transacetylase
Presence of lactose
Inactive
– Presence• Inactivation of
repressor• Expression
• Negative control