Chemistry of Life

Molecular genetics, biochemistry

gene regulation

Gene regulation mechanisms in eukaryotes, which possess a nuclear membrane, differ from those in prokaryotes.

attenuated operons : catabolite-regulated operons : eukaryotic regulation : lac operon : operator proteins, operon clusters, prokaryotic regulation : regulation by initiation : regulation by rate : simultaneous transcription translation : trp operon

Because prokaryotes lack a nuclear membrane, simultaneous translation of a gene may commence before transcription is complete. Prokaryotic genes are organized as operon clusters, and prokaryotic gene expression is controlled by activating or repressing the rate of transcription initiation.

Prokaryotes:
(a) long-term regulation of metabolism in bacteria is achieved through the control of initiation of transcription by such mechanisms as sigma factors, repressor proteins during induction and repression, and by the attenuation of many biosynthetic operons.

Control of prokaryotic gene expression is brought about by control of the rate of transcriptional initiation by two DNA promoter sequence elements that promote recognition of transcriptional start sites by RNA polymerase. Regulatory accessory proteins alter the activity of RNA polymerase at a given promoter by affecting the ability of RNA polymerase to recognize start-sites. These regulatory proteins can act both positively (activators) and negatively (repressors).

Proteins with sequences termed operators regulate the accessibility of promoter regions to prokaryotic DNA. The operator region is adjacent to the promoter elements in most operons, and in most cases the sequences of the operator bind a repressor protein. However, E. coli possesses several operons that contain overlapping sequence elements, one that binds a repressor and one that binds an activator.

Two major modes of transcriptional regulation in bacteria (E. coli) utilize repressor proteins to control the expression of operons.
1. Catabolite-regulated-operons employ repressor proteins to down-regulate operons that produce gene products necessary for the utilization of energy. A classic example of a catabolite-regulated operon is the lac operon, responsible for obtaining energy from β-galactosides such as lactose.
2. Attenuated operons regulate operons that produce gene products necessary for the biosynthesis of small biomolecules such as amino acids. Expression of the an attenuated operon class of operons is repressed by sequences within the transcribed RNA. A classic example of an attenuated operon is the trp operon, responsible for the biosynthesis of tryptophan.  Table gene regulation in E.coli .

In eukaryotes, control of gene expression is effected by a variety of mechanisms:
a. Most affect the rate of transcription by means of regulatory proteins that bind to regulatory sequences of DNA to bring about gene regulation.
b. Some alter the rate of RNA processing within the nucleus.
c. Some affect the stability and degradation of RNA molecules (nonsense-mediated decay, nonstop decay).
d. Some control the efficiency of ribosomal translation into ribosomal polypeptides and proteins.
e. Some allow for alternative splicing, which generates different proteins from the same archival DNA template.
f. Epigenetic mechanisms modify mRNAs.

Tables  Regulatory Proteins Sequences  Gene Regulation in E.coli  Genotype Variability  Cell Adhesion Molecules  Cell signaling  Complement Receptors  Cytokines  Eicosanoid Actions  Fc receptors  Receptor Tyrosine Kinases (RTKs)  Receptor Signal Transduction  Scavenger Receptors  Second Messengers  Toll-like Receptors  Phosphate-handling enzymes  Immune Cytokines  Electron Transport vs Oxidative Phosphorylation  Enzymes Function Krebs Cycle  Enzymes Cofactors of Krebs Cycle  Apoptosis vs Necrosis  Apoptosis 
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. . . transcription begun 10/06/06