Microbial Genetics

Bacterial Chromosome (DNA) Single Circular Attached one or many sites to plasma membrane

Bacteria chromosome Escherichia coli 4.6 million base pairs 4300 genes 1mm long 1,000 X length of cell Supercoiled Topoisomerase II DNA gyrase

Bacterial chromosome Genetic map Mapped in minutes Based on time for chromosome exchanged between two cells

Review

Eukaryotic differences Transcription takes place in nucleus mRNA completed prior to entry in cytoplasm Exons – Expressed DNA, code for protein Introns – intervening DNA, do not code for protein Removed by ribozymes

Regulation of Bacterial Gene Expression All metabolic reactions are catalyzed by enzymes (proteins) Feedback inhibition stops a cell from performing unneeded chemical reactions Stops enzymes that are already synthesized What prevents synthesis of enzymes that are not needed?

Regulation of Bacterial Gene Expression Protein synthesis requires tremendous energy Cell does not waste energy Regulating protein synthesis economizes cells energy

Regulation of Bacterial Gene Expression Genes 60-80% are constitutive Not regulated Products produced at fixed rate Genes turned on all the time Code for enzymes essential to major life processes Enzymes needed for glycolysis

Regulation of Bacterial Gene Expression Genes Inducible genes Production of enzymes is regulated Inducible enzymes Present only when needed

Regulation of Bacterial Gene Expression Regulation of transcription Repression Decreases gene expression Decrease enzyme synthesis Response to overabundance of an end product Regulatory proteins called repressors Block RNA polymerase

Regulation of Bacterial Gene Expression Regulation of transcription Induction Turns on genes Substance that turns on gene Inducer Inducible enzymes

Regulation of Bacterial Gene Expression Induction enzymes β-galactosidase (E. coli) Cleaves lactose Medium without lactose = little to no β-galactosidase Lactose added to medium large amounts of β-galactosidase produced Lactose is converted to allolactose Allolactose is the inducer Enzyme reduction

Operon Model Three genes for lactose utilization Located next to each other on bacterial chromosome Regulated together Called structural genes lac structural enzymes are transcribed and translated lac for lactose

Operon Model Operon model lac operon Promoter region Operator region Region of DNA where RNA polymerase initiates transcription Operator region Go or stop signal for transcription of the structural genes Structural genes Genes for metabolism of lactose

Lactose regulation Lactose operon Depends on level of glucose in medium Enzymes for glucose metabolism are constitutive When glucose is absent cAMP (cyclic AMP) accumulates in cell cAMP binds to cAMP receptor protein (CRP) This binds to lac promoter Initiates transcription by allowing mRNA polymerase to bind to the promoter Transcription of lac operon requires Presence of lactose Absence of glucose cAMP is an alarmone Chemical alarm signal the cell uses to respond to environmental or nutritional stress

Mutation Mutation Change in the base sequence of DNA may cause change in the product coded by the gene Beneficial Lethal Neutral Occur commonly Degeneracy

Mutations Types of mutations Base substitution (point mutation) AT substituted for CG mRNA carries incorrect base Translation Insertion of incorrect amino acid into protein Missense mutation, nonsense mutation, frame shift mutation, and spontaneous mutations

Base substitution

Mutation frequency Mutation rate Probability that a gene will mutate when a cell divides Expressed in power of 10 10-4 mutation rate (1 in 10,000 chance of mutation) 10-6 ( 1 in 1,000,000) Mutagens Increase spontaneous mutation by 10 – 10,000 times 10-6 becomes 10-3 to 10-5

Genetic Transfer and Recombination Genetic recombination Exchange of genes between two DNA molecules to form new combinations of genes on a chromosome Crossing over Two chromosomes break and rejoin Adds to genetic diversity

Genetic transfer and recombination Eukaryotes Meiosis Prophase I Prokaryotes Numerous different ways

Genetic Transfer and Recombination Vertical gene transfer Genetic information passed from an organism to its offspring Plants and animals Horizontal gene transfer Bacteria transfer genetic information form one organism to another in the same generation Genetic information passed laterally

Horizontal Gene Transfer Donor cell Organism gives up its entire DNA Part goes to recipient cell Part is degraded by cellular enzymes Recipient cell Receives portion of donor cells DNA Incorporates donor DNA into its own DNA Recombinant DNA Less than 1 % of population

Transformation Genes transferred from one bacterium to another in solution Naked DNA Discovered by Griffith Used Streptococcus pneumoniae Two strains Virulent (pathologic) strain Had a polysaccharide capsule resists phagocytosis Avirulent (non- pathogenic) strain Lacked a capsule

Transformation Bacteria after cell death and lysis could release DNA into environment Recipient cell can take up DNA fragments and incorporate into their own DNA Resulting in a hybrid (recombinant cell) Recombinant cell must be competent Able to alter cell wall to allow DNA (large molecule) to enter Bacillus, Haemophilus, Neisseria, Acinetobacter, and some Staph and Strep

Genetic Transformation

Conjugation Conjugation Involves plasmid Requires cell to cell contact Circular piece of DNA Replicates independent of chromosome Non essential for growth genes Requires cell to cell contact Opposite mating type Donor cell carries plasmid Recipient cell lacks plasmid

Conjugation Gram positive Gram negative Sticky surfaces cause bacteria to come in contact with one another Gram negative Utilize sex pili

Transduction in Bacteria Transfer of bacterial DNA transferred via bacteriophage Bacteriophage Virus that infects bacteria

Plasmids Plasmids Self replicating rings of DNA 1-5% size of chromosomal DNA Non – essential genes Conjugative plasmid F factor Dissimilation plasmids Code for enzymes to breakdown unusual sugars and hydrocarbons Help in survival of unusual environments

Plasmids Other plasmids Toxins (Anthrax, tetanus, Staph) Bacterial attachment Bacteriocins Toxic proteins that kill other bacteria Resistance factors (R factors) Resistance to antibiotics, heavy metals, cellular toxins

Plasmids Resistance factors Two groups RTF – resistance transfer factor Includes genes for plasmid replication and conjugation r-determinant Resistance genes Codes for production of enzymes that inactivate drugs or toxic substances Bacteria can conjugate and transfer plasmids between species Neisseria Penicillinase resists penicillin

R factor Plasmids