Transcription: TF, promoters, polymerases

Question 1

lac operon

Answer 1

In the absence of lactose:
Lac repressor (tetrameric protein) occupies the operator
Binding of RNA polymerase to the promoter is excluded
Lactose metabolizing genes (ZYA) are not transcribed.

In the presence of lactose:
Lactose binds to repressor
Affinity of repressor for operator is reduced–> lactose metabolizing genes are transcribed.

lactose metabolizing genes are found in sequence =polycistronic
Lactose permease- designated as Y (transport)
beta- galactosidase- Z (lactose –> galactose + glucose)
Galactoside transacetylase- A (nonmetabolites removal)

Question 2

Trp operon

Answer 2

Trp only produced in E. coli when it is not in the medium.

without Trp, aporepressor is not attached to the operon–> RNA polymerase can bind–> transcription is allowed.

with Trp–> Trp binds to the aporepressor–> become the holoenzyme–> bind to the operon and blocks RNA polymerase –> no transcription

absence of a product leading to activation of a synthetic pathway.

Question 3

Catabolite repression of lactose by glucose

Answer 3

glucose represes lactose metabolizing enzymes

cAMP levels are low in the presence of glucose.

cAMP is required for activation of DNA binding protein CAP (Catabolite Activator Protein ) and transcription of ‘lac’ operon.

when [glucose]=low=> increased levels of cAMP–> activate CAP–> transcription of lac operon.

lac operon will only be active if lactose is present and glucose is absent.

Question 4

Von Hippel-Lindau Disease (VHL)

Answer 4

rare, multi-system genetic disorder.
phenotype: Growth of tumors in parts of the body.
Hemangioblastomas - Tumors of the central nervous system
Angiomas – Tumors of the retina
Other types of tumors in the adrenal glands, kidneys, or pancreas.
Individuals with VHL are also at a higher risk for certain cancers, especially kidney cancer

ubiquitin ligase complex leads to the degradation of hypoxia inducible factor (HIF), a transcription factor.

VHL binds to elongin B and C components of the E3 ubiquitin ligase complex.

Loss of VHL leads to activation of angiogenic pathways–> tumor growth

HIF stimulates angiogenic pathways. In normal cells VHL interaction with elongin BC of ubiquitin ligase leads to degradation of HIF and suppression of angiogenic pathways

VHL mutations prevent its binding to elongin BC and thus inhibit the formation of active ubiquitin ligase.
HIF is not degraded
Angiogenic pathways are stimulated
Tumor growth is sustained

Question 5

Growth Hormone deficiency

Answer 5

Pit-1 transcription factor and GH (growth hormone)
Pituitary transcription factor (POU-Homeo protein) for transcription of growth hormone gene
PROP-1 and Pit-1
PROP-1 (Prophet of Pit-1) is also a homeo box protein
Transcription factor for Pit-1 gene
Mutations in PROP-1 or Pit-1 lead to GH deficiency

Mutations in Pit-1 prevent Growth Hormone gene transcription

Mutations in PROP-1 prevent Pit-1 transcription and consequently
Growth Hormone gene transcription

GH deficiency is linked to short stature–> begins at the level of transcription

Question 6

Huntington’s Disease

Answer 6

Huntington’s disease (HD) is an inherited neurodegenerative disease.
Progressive deterioration of motor and cognitive abilities.
Autosomal dominant disease
Mapping to chromosome 4
Molecular defect:
Expansion of CAG (glutamine) repeats

What does the glutamine expansion in huntingtin protein do?
May interfere with transcription in neurons.
Huntingtin protein is proposed to work via Sp1 transcription factor
Sp1 has a glutamine-rich activation domain
Sp1 interacts with a glutamine-rich factor TAFII 130 associated with TFIID.

HD brains have a deregulated transcription of specific genes.
Mutant protein may interfere with transcription in neurons.
Mutant huntingtin protein decreases transcription of several neuron specific genes including dopamine D2 receptor*****
Transcriptional dysregulation may be one of the earliest steps in HD etiology.