Regulation of gene expression Premedical - Biology

Regulation of gene expression in prokaryotic cell - Operon units, system of negative feedback in eukaryotic cell - at any stage of gene expression and proteosynthesis. Noncoding RNAs are important.

Operon model is a functional unit common in bacteria and phages. A ctivation and inhibition of transcription are regulated in response of conditions in environment. Prokaryotic genes do not have introns and exons.

Operon coordinately regulated clusters of genes, which are transcribed into one mRNA (polygenic mRNA, polycystron transcript) genes for particular metabolic pathway, regulated by common promotor and ordered on DNA following each other

Escherichia coli Lac operon, Trp operon – model systems = metabolic pathways of utilization of lactose gen lacZ, lacY, lacA, catabolic pathway with negative and positive regulation enzymes for TRP synthesis, anabolic pathway with negative regulation

each operon consists of promoter (for RNA polymerase) operator (for repressor) several structural genes terminator repressor = allosteric protein encoded by regulatory gene co-repressor = product molecule inducer = substrate molecule

Tryptophan operon

Lac operon - negative regulation regulatory gen produces repressor, which binds operator and causes that RNAP is not able to initialize transcription in the presence of lactose repressor is released from operator. The repressor is changed by inducer / lactose RNA polymerase starts the transcription. In 2-3 minutes the amount of enzymes is increased 1000x

Lac operon - negative regulation

In the presence of glucose, E. coli preferentially uses glucose for decomposing. If is low level of glucosis, the cAMP is increased. CAP „Catabolite activator protein“ in the presence of cAMP attaches promotor and activates the transcription. CAP is allosteric regulatory protein Lac operon - positive regulation

Summary : Lac operon is active only in time, when the activator CAP+cAMP is attached on promotor (no glucose) and when is not present represor on operator (lactose present) Lac operon - positive regulation

Gene expression of eukaryotic cells each cell maintains specific program / differential gene expression one mRNA carries information for one gene (monogenic mRNA) posttranscription modifications of RNA, RNA processing and splicing regulation system is performed at several levels = transcription, translation, protein activation + secretion

Six steps at which eucaryotic gene expression can be controlled

chromatin changes transcription processing RNA transport to cytoplasm degradation of mRNA translation cleavage, chemical modification protein degradation more complicated regulative system

Stages in gene expression in eukaryotic cell

Heterochromatin is highly condensed, that is why transcriptional enzymes can not reach the DNA Acetylation / deacetylation of histons Methylation [cytosin] - inactive DNA is highly methylated DNA methylation and histone de-acetylation repress the transcription. 1. Chromatin changes

DNA methylation is esential for long-term inactivation of genes during cell differentiation Gene imprinting in mamals methylation constantly turns off the maternal or the paternal allele of a gene in early development certain genes are expressed in a parent-of-origin- specific manner Epigenetic regulation

2. Transcription Transtcribtion factors - proteins that bind DNA and facilitate or inhibit RNA polymerase to bind. They are a part of transcription initiation complex. Transcription factors: general transcription factors for all protein-coding genes specific transcription factors – transcription of particular genes at appropriate time and place - enhancers, activators, inhibitors, repressors

Eukaryotic gene and transcript

Cell-type specific transcription: Genes encoding enzymes of one metabolic pathway are scattered over the different chromosomes - coordinated control in response of chemical signals from outside the cell. The cell accepts signals by receptors. Signal transduction pathways activate transcription activators or repressors.

Signal transduction pathways

3. Processing RNA Post-transcriptional modifications Alternative splicing The same primary transcript, but different mRNA molecule from it (different exons and introns) 4, 5. transport of mRNA / degradation Lifespan of mRNA is important for protein synthesis.

6. Translation At the initiation stage – regulatory proteins bind the 5’ end of mRNA with the cap. Activation or inactivation of protein factors to initiate translation

7. Cleavage, chemical modifications Cleavage Post-translational modifications Regulatory proteins [products] are activated or inactivated by reversible addition of phosphate groups / phosphorylation Proteins for cell surface need sugars / glycosylation

Polypeptide chain may be cleaved into two or three pieces Preproinsulin Proinsulin - disulfide bridges Insulin Secretory protein

Acid/base - act/inact Hydrolysis – localization, act/inact Acetylation - act/inact Phosphorylation - act/inact Prenylation - localization Glycosylation - targeting Post-translational modifications

Various steps in the synthesis and assembly of collagen fibrils

8. protein degradation Lifespan of protein is strictly regulated Protein for destruction is attached to a small protein ubiquitin. Protein complexes Proteasomes are places of degradation.

Thank you for your attention Campbell, Neil A., Reece, Jane B., Cain Michael L., Jackson, Robert B., Minorsky, Peter V., Biology, Benjamin-Cummings Publishing Company, 1996 –2010.