lecture 11 - Bacteria and gene regulation in prokaryotes

Describe bacteria?

• Unicellular

• 3 common shapes

• Spheres (cocci), rods (bacilli) and spirals

Describe the roles of bacteria in the environment?

• Parasites - plants and animals

• Mutualist - in our guts

• Saprophytes - help with degradation of organic matter

Describe the structure of bacteria?

• Lack membrane, no nucleus

• DNA floats within cytoplasm

• Causes translation and transcription

Describe gene expression in bacteria?

• Since bacteria have no nucleus, transcription and translation occur in the same place

Describe transcription of mRNA in prokaryotic vs eukaryotic?

• Prokaryotic - transcription and translation happen in cytoplasm

• Eukaryotic- transcription happens nucleus, translation happens in cytoplasm

Describe how gene expression is regulated?

• Bacteria respond to environmental change by regulating gene expression

Describe molecular anatomy of a gene (bacteria)?

• Gene has two parts, coding and promoter region

• Bacteria transcription done by single RNA polymerase

• RNA polymerase has 5 subunits and a regulatory sigma subunit

• Sigma subunit required to transcription happens in correct manner

Describe the operon?

• Unit of genetic function

• Cluster of genes with related function

• Operons composed of promoter, operator (on off switch), and genes for proteins that work together

• All genes can be switched on and off by regulatory proteins

Describe the structure of genes?

• Region of transcriptional control consist of promoter and operator, followed by genes (polycistronic mRNA)

• Promoter - specific DNA sequence recognised by RNA polymerase, RNA polymerase binds to promoter and transcription is initiated

• Operator - specific DNA sequence, can turn a gene on or off

Describe a lac operon?

Genes for lactose are

• clustered together on chromosome into operon

• Controlled by single promoter

• Transcribed as a single transcript

• There is a control region, operator, followed by LacZ, LacY, LacA

• Another gene called Lacl with own promoter

• Lacl promoter codes for mRNA, creating inhibitors, binding to operator

What happens when lactose is absent?

• mRNA from Lacl forms inhibitor which binds to operator

• Inhibits transcription of LacZ, LacY, LacA

What happens when lactose is present?

• A lactose isomer (allolactose) binds to inhibitor

• Inhibitor changed shape

• Inhibitor cant bind to operator anymore

• Therefore can transcribe all genes (LacY, LacZ, LacA)

Describe catabolite repression?

• If glucose is present

• E coli prefer glucose than lactose as not much energy is spent

• Use glucose first (high growth)

Describe a strong activation of lac operon?

• 2 conditions need to be met

• Lactose must be present to prevent inhibitor binding on operator

• Level of cyclic AMP must be high (positive control)

• If 2 conditions are met, then cell is signalled that lactose is present and glucose is absent

Describe activation of Lac operon

1. Low glucose levels mean high levels of cyclic AMP

2. Cyclic AMP binds to CAP, activating it

3. Active CAP binds to the activator region of DNA, increasing RNA polymerase activity

Describe what happens if lactose and glucose is high?

• High glucose results in low levels of cyclic AMP

• Thus AMP less likely to bind to CAP

• Little active CAP, thus RNA polymerase activity is low

Describe Tryptophan synthesis?

• Starts with precursor molecule

Converted step by step

• Enzyme 1 - trp d trp e

• Enzyme 2 - trp c

• Enzyme 3 - trp a

• If tryptophan is abundant, can regulate in 2 ways

• 1 - block enzyme 1, therefore pathway is stopped

• 2 - tryptophan can control operator when needed to control 5 genes

What happens if tryptophan is absent?

• Operator is on

• RNA polymerase start to make mRNA from 5’ to 3’

What happens when tryptophan is present?

• Tryptophan binds to operator anymore

• No RNA activity

Describe tryptophan by default?

• By default, trp operan is on, and genes are transcribed

• When there is too much trp, it binds to operator and turns it off

• When tryptophan is turns off, means high levels of tryptophan