Regulation of gene expression Premedical - biology

Regulation of gene expression in prokaryotic cell - Operon systems, negative feedback in eukaryotic cell – regulated at any stage, noncoding RNAs

Operon model  a unit of genetic function common in bacteria and phages  activation and inhibition of transcription in response of enviroment – changes in the metabolic status of the cell  genetic information is not divided into introns and exons

Operon coordinately regulated clusters of genes, which are transcribed into one mRNA (polygennic mRNA, polycystron transcript) genes for particular metabolic pathways are regulated by common promotore and are ordered onto DNA followed by each other

Escherichia coli Lac operon, Trp operon – model systems utilisation of lactosis gen lacZ, lacY, lacA, inducible enzymes / catabolic pathway negative and positive regulation enzymes for TRP synthesis, repressible enzymes / anabolic pathway negative regulation / switching off

Operon promoter (RNA polymerase is bound) operator (repressor is bound) several structural genes terminator repressor – regulatory gene, allosteric protein corepressor – product molecule inducer – substrate molecule

Tryptophan operon

Lac operon - negative regulation regulatory gen produces repressor, binds oprator and causes that RNAP is not able to inicializate of transcription in the presence of lactosis, represor releases operator, low molecular molecule is called inducer RNA polymerase starts the transcription. In 2-3 minutes the amount of b- galactosidase increase 1000x

Lac operon - negative regulation

In the presence of glucose, E. coli preferentially uses glucose Allosteric regulatory protein „Catabolite activator protein“ CAP in the presence of cAMP attaches promotor and activates The activity of gene only in presence of cAMP – low level of glucosis Lac operon - positive regulation

Summary : Lac operon is active only in time, when the activator CAP+cAMP is attached onto promotor, but when is not present represor onto operator Lac operon - positive regulation

Lac - operon 1.High level of glucosis, not present lactosis, negative regulation on and there is not a positive regulation 2.High level glucosis and lactosis, negative regulation and positive regulation are off, lac operon is not active 3.In cells is a low level of glucosis and high level of lactosis, there is not negative regulation and there is positive regulation, all enzymes are in very high amount 4.Low level of glucosis and lactosis – not active

Gene expression of eukaryotic cells each cell maintain specific program / differential gene expression one mRNA carries information for one gene (monogennic mRNA) posttranscription modification of RNA (removing introns and connecting exons) complicated regulation system, performed at the several levels (transcription, translation, protein activation + secretion)

chromatin changes transcription processing RNA transport to cytoplasm degradation of mRNA translation cleavage, chemical modification protein degradation Complicated regulation system

Stages in gene expression in eukaryotic cell

Six steps at which eucaryotic gene expression can be controlled

Heterochromatin is highly condensed - transcriptional enzymes can not reach the DNA Acetylation / deacetylation of histons Methylation [cytosin] - inactive DNA is highly methylated DNA methylation and histone deacetylation repress 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 inheritance

2. Transcription proteins that bind to DNA and facilitate of inhibit binding of RNA polymerase transcription initiation complex transcription factors – general transcription factors for all protein-coding genes - specific transcription factors – transcription of particular genes at the appropriate time and place - enhancers, promoters associated with a gene

Eukaryotic gene and transcript

Cell-type specific transcription Genes coding for the enzymes of a metabolic pathway are scattered over different chromosomes - coordinated control in response of chemical signals from outside the cell - receptors signal transduction pathways activating of transcription activators or repressors

Signal transduction pathways

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

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

7. Cleavage, chemical modification Cleavage Post-translational modifications Regulatory proteins [products] are activated or inactivated by the reversible addition of phosphate groups / phosphorylation Sugars on surface of the cell / 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 Marked protein for destruction is attached by a small protein ubiquitin Protein complexes proteasomes

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.