Gene expression

1. REGULATION OF GENE
EXPRESSION IN PROKARYOTES

2.  Regulation of protein synthesis is necessary in all
cells.
 Particularly, in the prokaryotic cells, regulation is
achieved by means of genetic units called operons
alongside a number of related mechanisms as
described below:

3. Regulation of Protein Synthesis on the
Basis of Nutrient Supply
 Prokaryotes respond to changes in their supply of
nutrients in a way that allows them to obtain or conserve
energy most efficiently.
 For Example.
 E. coli uses glucose preferentially whenever it is
available. The enzymes in the pathways for glucose
utilization are made constitutively i.e., they are
constantly being produced.
 However, if glucose is not present in the medium but
another sugar is available, E. coli produces the enzymes
and other proteins that allow the cell to derive energy
from that sugar

5. 2.Regulation by means of Operons
 Regulation of protein synthesis in prokaryotes occurs
mainly at the transcriptional level, and involves genetic
units known as operons.
 An operon is a set of genes that are adjacent to one
another in the genome and are coordinately controlled;
that is, the genes are either all turned on or all turned off.
 Operons contain promoter regions where proteins bind
and facilitate or inhibit the binding of RNA polymerase.
 When RNA polymerase transcribes the structural genes
of an operon, a polycistronic mRNA (i.e., an mRNA that
codes for more than one polypeptide) is produced.

6. 2.1. Regulation by Induction
 Induction is the process whereby an inducer (a small molecule) stimulates the
transcription of an operon.
 The inducer is frequently a sugar (or a metabolite of the sugar), and the proteins
produced from the inducible operon allow the sugar to be metabolized.
 Working of an Inducer:
 (1) The inducer binds to the repressor, inactivating it.
 (2) The inactive repressor does not bind to the operator.
 (3) RNA polymerase, therefore, can bind to the promoter and transcribe the operon.
 (4) The structural proteins encoded by the operon are produced.
 Example:
 If glucose is not present in the provided medium but another sugar is available, E.
coli produces the enzymes and other proteins that allow the cell to derive energy
from that sugar.
 The process by which the synthesis of the enzymes is regulated is called induction.
 The Lac operon is inducible.

7. 2.2. Regulation by Repression
 Repression is the process whereby a corepressor (a small molecule) inhibits
the transcription of an operon.
 The co-repressor is usually an amino acid, and the proteins produced from
the repressible operon are involved in the synthesis of the amino acid.
 Working of a Repressor
 (1) The corepressor binds to the repressor, activating it.
 (2) The active repressor binds to the operator.
 (3) RNA polymerase, therefore, cannot bind to the promoter, and the
operon is not transcribed.
 (4) The cell stops producing the structural proteins encoded by the operon.
 For Example:
 If an amino acid is present in the medium, E. coli does not need to
synthesize that amino acid and conserves energy by ceasing to produce the
enzymes required for its synthesis. The process by which the synthesis of
these enzymes is regulated is called repression.
 The Tryptophan operon is repressible.

8. 3.Regulation by Positive Control
 Some operons are turned on by mechanisms that
activate transcription.
 When the repressor of the arabinose (ara) operon
binds arabinose, it changes the conformation and
becomes an activator that stimulates the binding of
RNA polymerase to the promoter.
 The operon is then transcribed, and the proteins
required for the oxidation of arabinose are produced.

9. 4.Regulation by Catabolite Repression
 Some operons (e.g., lac and ara) are not expressed when glucose is present
in the medium. These operons require cAMP for their expression.
 Working:
 (1) Glucose causes cAMP levels in the cells to decrease.
 (2) When glucose decreases, cAMP levels rise.
 (3) cAMP binds to the catabolite-activator protein (CAP).
 (4) The cAMP–protein complex binds to a site near the promoter of the
operon and facilitates
 binding of RNA polymerase to the promoter.
 Example:
 The lac operon exhibits catabolite repression.In the presence of lactose and
the absence of glucose, the lac repressor is inactivated, and the high levels
of cAMP facilitate the binding of RNA polymerase to the promoter. The
operon is transcribed, and the proteins that allow the cells to utilize lactose
are produced.

10. 5.Regulation by Attenuation
 In bacterial cells, transcription and translation occur
simultaneously.
 Attenuation occurs by a mechanism by which rapid
translation of the nascent transcript causes the termination of
transcription.
 As the transcript is being produced, if ribosomes attach and
rapidly translate the transcript, a secondary structure is
generated in the mRNA that is a termination signal for RNA
polymerase.
 If translation is slow, this termination structure does not
form, and transcription continues.
 Example:
 The trp operon, as well as other amino acid biosynthetic
operons, is regulated by attenuation.

11. References
 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC19
6965/
 https://microbenotes.com/regulation-of-protein-
synthesis-in-prokaryotes/