Regulation of Protein Synthesis

Prokaryotes

Regulation of translationGene expression is regulated not only on

the transcription level but it is also regulated in translation level.

One advantage of control of translation over transcription is the ability to respond very rapidly to external a stimuli.

As with other types of regulation, translational control typically functions at the level of initiation.

Protein or RNA Binding near the Ribosome-Binding SiteNegatively Regulates Bacterial Translation Initiation

Ribosomal Proteins Are Translational Repressors of Their Own Synthesis

STRINGENT RESPONSEA Global translational control

When bacteria are starved of nutrients, they immediately shut down gene expression and other metabolic activities

Total RNA synthesis is reduced to ~ 10% of normal levels.

There is a massive >10-fold reduction in rRNA and tRNA transcription.

Protein synthesis decreases.

(The unusual nucleotides ppGpp and pppGpp accumulateduring the stringent response).

RelA, SpoT key stringent factors

pppGpp-inhibit transcriptionStringent factors are pppGpp synthetase and

produce one everytime the A-site or ribosome is occupied by an uncharged tRNA.

The stringent factor RelA is a pppGpp synthetase that isassociated with ~ 5% of ribosomes.

Ribosomal protein L11 undergoes a conformational change when an uncharged tRNA binds and then activate stringent factors.

pppGpp inhibit the transcription elongation by inhibiting the RNA polymerase.

pppGpp Guanosine pentaphospate

Stringent response in E. coli

Binding of an unchargedtRNA to the A-site

Binding of RelA to the 30S subunit

Synthesis of ppGppDownregulation /

inhibition oftranscription

Riboswitch Riboswitch is a regulatory segment of a

messenger RNA molecule that binds a small molecule, resulting in a change in production of the proteins encoded by the mRNA

It can either represses or enhance the mRNA translation and also act on transcription level.

Riboswitches are often conceptually divided into two parts: an aptamer and an expression platform.

Two platformsThe aptamer directly binds the small molecule, and the expression platform undergoes structural changes in response to the changes in the aptamer. The expression platform is what regulates gene expression.

• Transcription termination and anti-termination

• Translation initiation: RBS accessibility

• RNA processing:Splicing or degradation

Riboswitch mechanisms

Glucosamine-6-phosphate Riboswitch

Tom Cech, Nature (2004) 428: 263-264.

Translation-dependent regulation of mRNA and protein stability At some frequency, mRNAs will be made

that are mutant or damaged

Such damaged mRNAs have the possibility of making incomplete or incorrect proteins that could have negative effects on the cell

the process of translation is used to detect defective mRNAs and eliminate them and their protein products.

The SsrA RNA Rescues Ribosomes That Translate Broken mRNAs

When in broken mRNAs translation the ribosome is unable to move out of the translation complex.

In prokaryotic cells, such stalled ribosomes are rescued by the action of a chimeric RNA molecule that is part tRNA and part mRNA, appropriately called a tmRNA• SsrA is a 457-nucleotide

tmRNA that includes a region at its 30 end that strongly resembles tRNAAla

• SsrA RNA acts as an mRNA and

encodes 10 codons followed by a stop codon

Inhibition of protein synthesis by antibiotics

ReferenceVoet, D. & Voet, J. G. (2011) “ Biochemistry”.

John Wiley & Sons. INC.Watson, et. al. (2014) “Molecular Biology of

The Gene”. Seventh Edition. Pearson Education, Inc.

Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2000). Lehninger principles of biochemistry. New York: Worth Publishers.