Today:• In-Class 5 (Telomere) Wrap-up• RNAi Overview and Discussion • Preview: Regulating Gene Expression •Exam Return

Reviewing RNAi

Introduction to Regulating Gene Expression in Eukaryotes

In eukaryotes, gene regulation functions primarily by either enhancing or restricting RNA Polymerase’s access to promoters

Introduction to Regulating Gene Expression in Eukaryotes

Eukaryotes must be able to create enormous diversity in their expression patterns. Two primary mechanisms accomplish this:

1.promoter-proximal elements (transcription factors near the promoter)- tend to affect expression of many genes

2.enhancers or upstream activation sequences- tend to more specifically target a smaller subset of genes

Introduction to Regulating Gene Expression in Eukaryotes

The Model: Saccharomycese cerevisiae

Introduction to Regulating Gene Expression in Eukaryotes

• Key Regulator is Gal4 Protein• Gal4 regulates the expression of

GAL1, GAL2, GAL7, and GAL10• Each of these genes has 2+, 17 bp

Gal4 binding sites (enhancers) located upstream of its promoter

Gal4 Function

Experimental work with reporter genes demonstrates that two critical domains, the DNA-binding domain, and the activation domain, are both required for Gal4 function.

Gal4 Regulation

A second set of proteins regulates Gal4.

Gal80 normally blocks Gal4 function, unless Gal3 inactivates it.

Gal3 only binds to Gal80 when it is bound to galactose!

Gal4 Activity

Gal4, like many activators, can both attract other proteins involved in initiating transcription and recruit proteins to modify chromatin structure.

Chromatin Remodeling

Promoters wound in nucleosomes are not accessible to the RNA

polymerase!

Chromatin Remodeling

The histone tails of histone proteins can be modified by the covalent attachment of a acetyl or methyl group.

Chromatin Remodeling

Acetylation can make an octomer more likely to slide along the DNA and can influence the binding of regulatory proteins.

In general histone acetylation promotes gene transcription.

Enhancesomes

Enhancesomes are complexes of regulatory proteins that synergistically activates transcription to very high levels when all members are present.

Example: Beta-interferon

Combinatorial Control in Yeast: the MAT Locus

Two haploid and one diploid mating types are known in yeast. Interestingly, cells can switch mating types!

Despite mating type being controlled by a single locus, each type expresses a unique set of genes.

Combinatorial Control in Yeast: the MAT Locus

Enhancer-Blocking Insulators

• Prevent an enhancer from activating transcription when positioned between an enhancer and promoter

Imprinting

• Imprinting explains unusual inheritance patterns of autosomal genes in mammals

• In paternal imprinting, the paternal copy is inactive; in maternal imprinting the maternal copy is inactive

Callipyge sheep 1 and 3 are descendents of Solid Gold. Sheep 2 and 4 are normal. Courtesy Sam P. Jackson

• Imprinting occurs because e DNA is methylated in sex-specific manner during the development of the gametes.

• These methylation patterns can be stably inherited.

• At a molecular level, methylation can prevent the binding of insulators.

Imprinting

• Chromatin condensation patterns change during the course of the cell cycle

• Heterochromatin contains few genes, while euchromatin is rich in genes

The Genomic Neighborhood

• In fruit flies, chromosomal rearrangements can move an eye pigment gene to a region of heterochromatin, an example of epigenetic silencing.

• When nearby, heterochromatin can spread into euchromatin, silencing genes. This is known as position-effect variation (PEV).

The Genomic Neighborhood

• X-inactivation allows for dosage compensation of the genes on the X chromosomes in mammals.

• This is accomplished through the formation of a Barr body (DNA and histones associated with Barr bodies are heavily methylated!)

X-Inactivation

• Read Chapter 11 (Use your worksheet and these notes to guide you!

• Take your quiz (Mon-Wed)• Use your discussion room!• Read the Hypermethylome paper and

prepare your discussion questions• Bring a draft of your poster layout

For Next Week