Control of Gene Expression

Eukaryotes

Eukaryotic Gene Expression

• Some genes are expressed in all cells all the time. These so-called housekeeping genes are responsible for the routine metabolic functions (e.g. respiration) common to all cells.• Some are expressed as a cell enters a particular

pathway of differentiation.• Some are expressed all the time in only those cells

that have differentiated in a particular way. For example, a plasma cell expresses continuously the gene for the antibody it synthesizes.• Some are expressed only as conditions around and in

the cell change. For example, the arrival of a hormone may turn on (or off) certain genes in that cell

One Gene = One Protein Hypothesis

• By Beadle and Tatum (1940’s)• The one gene-one enzyme

hypothesis is the idea that genes act through the production of enzymes, with each gene responsible for producing a single enzyme that in turn affects a single step in a metabolic pathway.• http://www.dnalc.org/view/

16360-Animation-16-One-gene-makes-one-protein-.html

The Eukaryotic Gene • Associated with most eukaryotic genes are control elements, segments of

noncoding DNA that help regulate transcription by binding certain proteins• Control elements and the proteins they bind are critical to the precise regulation

of gene expression in different cell types

Many Levels of Control in Eukaryotes

• Chromatin Remodeling The region of the chromosome must be opened up in order for enzymes and transcription factors to access the gene

• Transcription Control The most common type of genetic regulation

• Turning on and off of mRNA formation• Post-Transcriptional Control Regulation of the

processing of a pre-mRNA into a mature mRNA• Translational Control Regulation of the rate of Initiation• Post-Tranlational Control (protein activity control)

Regulation of the modification of an immature or inactive protein to form an active protein

Chromatin Remodeling

• Genes within highly packed heterochromatin are usually not expressed• Chemical modifications to

histones and DNA of chromatin influence both chromatin structure and gene expression• Requires ATP

Chromatin Remodeling

• In histone acetylation, acetyl groups are attached to positively charged lysines in histone tails, which neutralizes them• The addition of methyl groups

(methylation) can condense chromatin; the addition of phosphate groups (phosphorylation) next to a methylated amino acid can loosen chromatin

Chromatin Remodeling

Transcription Control

• To initiate transcription, eukaryotic RNA polymerase requires the assistance of proteins called transcription factors• In eukaryotes, high levels of transcription of particular genes depend on

control elements interacting with specific transcription factors• Proximal control elements are located close to the promoter• Distal control elements, groups of which are called enhancers, may be

far away from a gene or even located in an intron• An activator is a protein that binds to an enhancer and stimulates

transcription of a gene

Transcriptional Control

Transcriptional Control • Methylation of bases also turns off transcription• DNA methylation can cause long-term

inactivation of genes in cellular differentiation• In genomic imprinting, methylation regulates

expression of either the maternal or paternal alleles of certain genes at the start of development• Although the chromatin modifications just

discussed do not alter DNA sequence, they may be passed to future generations of cells• The inheritance of traits transmitted by

mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance

Post-Transcriptional Control• Alternative RNA splicing: different mRNA molecules are produced

from the same primary transcript, depending on which exons are spliced out by the splicosome • mRNA degradation: without a 5’ cap and poly-A tail an mRNA will be

destroyed

Post-Translational Control (protein activity)

• After translation, various types of protein processing, including cleavage and the addition of chemical groups, are subject to control• Proteasomes are giant protein complexes that bind protein molecules and

degrade them• Ubiquitin's are placed on any protein as the signal for it to be degraded in the

proteasome

Noncoding RNAs

• Only a small fraction of DNA codes for proteins, rRNA, and tRNA• A significant amount of the genome

may be transcribed into noncoding RNAs• Noncoding RNAs regulate gene

expression at two points: mRNA translation and chromatin configuration

MicroRNAs

• MicroRNAs (miRNAs) are small single-stranded RNA molecules that can bind to mRNA• These can degrade mRNA or block

its translation (translational control)

siRNAs

• The phenomenon of inhibition of gene expression by RNA molecules is called RNA interference (RNAi)• RNAi is caused by small interfering RNAs

(siRNAs)• siRNAs play a role in heterochromatin

formation and can block large regions of the chromosome• Small RNAs may also block transcription of

specific genes

miRNA vs. siRNA

• both are formed from dsRNA and both eventually get cleaved into pieces by Dicer and then incorporated into RISC which in effect cleaves target mRNA.• siRNAs and miRNAs are similar but form from different RNA

precursors• Sirna Therapeutics, Inc. was a San Francisco, California based

biotechnology company that explores the use of RNA interference in human disease therapy. Sirna's development pipeline includes several small interfering RNA (siRNA) drugs, thought to stably silence the expression of specific disease-related genes.