6.1.1 (b) - regulatory mechanisms

spec points

the regulatory mechanisms that control gene expression at the transcriptional level, posttranscriptional level and post-translational level To include control at the:

  • transcriptional level: lac operon, and the general role of transcription factors in eukaryotes (Learners are not required to recall specific transcription factors)

  • post-transcriptional level: the editing of primary mRNA and the removal of introns to produce mature mRNA

  • post-translational level: the activation of proteins by cyclic AMP. HSW2

How are genes regulated?

  • different levels of control of gene expression

    • Transcriptional - genes are turned on/off.

    • Post-transcriptional - modification of mRNA.

    • Translational - stopping/starting translation.

    • Post-translational - modification of proteins after synthesis.

Chromatin remodelling:

  • Heterochromatin: tightly wound w/ histone, can’t be transcribed

  • Euchromatin: loosely wound, can be transcribed

Histone modification:

  • +charged histone attracts -charged DNA molecules to help w/ packaging of DNA

  • Acetylation (adding acetyl groups) to histone reduces +charge on histone, makes DNA coil less tightly coiled on histone → greater gene expression

  • Methylation of histone makes them bind DNA stronger → decreases gene expression 

  • Both are post-translational modifications of histone protein!

  • Can also be methylation/acetylation of cytosine/adenine bases in DNA.

  • These are epigenetic changes.

Transcription factors:

  • TF: short non-coding sections of RNA (or proteins) - bind at specific promoters on DNA + either promote/suppress transcription.

  • About 8% of genes code for transcription factors.

  • Promoters not always right next to structural gene when DNA unwound. Also, translocation mutations can separate promoters from their gene.

Operons in prokaryotes

The role of cyclic AMP

The Lac Operon: Gene induction

  • Prokaryotes: genes arranged in operons containing control regions + structural genes

  • Promoter: recognition site where RNA polymerase binds to DNA.

  • Operator: Potential blocking site where repressor protein may bind preventing RNA polymerase from binding.

    • so controls production of mRNA (transcription).

  • Structural genes: Code for particular functional protein e.g. enzyme/channel protein.

    • Beta-galactosidase (lac Z): hydrolysis of lactose → glucose and galactose

    • Lactose Permease (lac Y): increase uptake of lactose (transporter protein)

  • Regulatory gene (lacl)

    • Not part of operon

    • Codes for repressor protein that recognises + binds to operator. Affects/ prevents transcription.

  • Process is called gene induction

operon: promotor -> lactose permease

The Lac Operon: when no lactose is present

  • when no lactose present, genes need to be switched off - saves energy

The lac Operon: when lactose is present

starter review

  1. What are the 4 levels of control of gene expression?

    • translational, post-translational, transcriptional, post transcriptional,

  2. How does the strength of binding DNA to histone affect gene expression?

    • Stronger binding leads to lower levels of gene expression in that region.

  3. What type of molecules re produced by HOX genes?

    • Homeodomain protein, Transcription factor.

  4. In the lac operon, what is the operator?

    • Region of DNA (sequence of bases) in the operon where repressor protein can bind to prevent RNA polymerase from binding.

  5. What level of control is shown by the lac operon

    • Transcriptional

  6. What level of control is shown by phosphorylation of glycogen synthetase 

    • Post-translational: Phosphorylation de-activates the enzyme.

Lac operon in different conditions

  • high glucose, low lactose

    • no

  • high glucose, high lactose

    • yes

  • low glucose, low lactose

    • no

  • low glucose, high lactose

    • yes

will lac operon structural genes be transcribed and why/why not

Control of transcription: Role of cyclic AMP in lac operon

  • protein called cAMP receptor protein (CRP) is involved.

  • binding site for this protein next to promoter region of lac operon (where RNA polymerase binds).

  • When CRP protein binds to binding site it makes it easier for RNA polymerase to bind to promoter + so increases expression of lac operon.

  • CRP protein can only bind to binding site when it’s activated by binding to cAMP. (also an example of post-translational modification)

  • cAMP builds up when glucose levels low (glucose may be running out).

  • when glucose levels are high, cAMP levels are low, so CRP protein is inactive + can’t bind to binding site → RNA polymerase can’t bind as easily → expression of lac operon is reduced.

  • So Lac operon has on/off switch (induction) (presence of lactose (+ therefore if repressor binds or not) + also volume control (positive regulation) due to glucose conc

post transcription modification of mRNA

  • Pre-mRNA has to be modified to produce mature mRNA transcript.

  • Occurs in nucleus

    • 5’ cap added (nucleotide with guanine) - about getting RNA into ribosomes

    • Poly A tail added at 3’ end (lots of adenine nucleotides)

    • Delays degradation in cytoplasm + 5’ cap helps binding to ribosomes

    • Splicing to remove introns (exons joined together)

  • RNA editing (by deletion, insertion and substitution) creates different mRNA sequences so varies polypeptide primary structure.

  • all of this increases range of polypeptides possible

translational control

  • rate of translation can be varied too

  • varying rate of degradation of mRNA transcript

  • binding inhibitory proteins to mRNA preventing binding to ribsomes

  • activation of initiation factors - helps mRNA get into ribosome

    • (both last 2 affect 5’ cap)

  • RNA interference (MiRNA)

post-translational control

  • modification of protein that’s been synthesised

    • can add non protein groups

    • modifying AAs e.g. to form disulphide bridges

    • folding + shortening of proteins

    • can have modification by cAMP

  • role of protein kinases:

    phosphorylation of enzymes often involved in activation of them (changing shape as phosphate binds)

    e.g. role of cyclic AMP (2nd messenger system)

  • 2nd messenger which may also phosphorylate proteins (CREB/cAMP response element binding) that enter nucleus and act as transcription factors