1. DEPARTMENT OF MICROBIOLOGY AND IMMUNOLOGY
OPERON
MAHERUN NESA
RESIDENT (PHASE B)
DEPARTMENT OF MICROBIOLOGY AND IMMUNOLOGY
BSMMU

2. How do bacteria adapt so quickly to their environments?

3. Part of the answer to this question lies in clusters of co-regulated genes called operons.

4. Bacteria are typically exposed to an ever-changing environment in which nutrient availability may increase or decrease radically. Bacteria respond to such variations in their environment by altering their gene expression pattern; thus, they express different enzymes depending on the carbon sources and other nutrients available to them.

5. For example when lactose isn't around, the bacteria don't waste energy making proteins to break down lactose. When lactose is around, the bacteria waste no time in creating the proteins needed to break down lactose to get energy.

6. Bacterial genes are organized into operons, or clusters of co-regulated genes. These genes are regulated such that they are all turned on or off together. Grouping related genes under a common control mechanism allows bacteria to rapidly adapt to changes in the environment.

7. History
The term "operon" was first proposed in a short paper in the Proceedings of the French Academy of Science in From this paper, the general theory of the operon was developed.
The development of the concept is considered a landmark event in the history of molecular biology

8. Operons were first identified as a mode of gene expression control in 1961 by François Jacob and Jacques Monod.
In 1965 Nobel Prize in Physiology and Medicine was awarded to François Jacob, André Michel Lwoff and Jacques Monod for their discoveries concerning the operon and virus synthesis.

9. The first operon to be described was the lac operon in E. coli.
Operons are present in prokaryotes (bacteria and archaea).
Discovery of the first operons in eukaryotes in the early 1990s. Present in certain nematodes and the fruit fly, Drosophila melanogaster.

10. Operons are also found in viruses such as bacteriophages
Operons are also found in viruses such as bacteriophages. For example, T7 phages have two operons.
The first operon codes for various products, including a special T7 RNA polymerase which can bind to and transcribe the second operon.
The second operon includes a lysis gene meant to cause the host cell to burst.

11. Definition
Operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter.
Several genes must be co-transcribed to define an operon

12. Components of Operon There are 3 basic components to an operon -
Promoter
Operator
Structural genes

13. Structure of Operon

14. Promoter
Promoter is a upstream nucleotide sequence to which the RNA polymerase binds before the process of RNA transcription is initiated. RNA transcription leads to the synthesis of RNA which is then translated into functional proteins. These proteins may include enzymes, hormones, or other metabolites.

15. Operator
Segment of DNA to which a repressor protein binds and inhibits transcription (inhibits transcription by obstructing RNA polymerase to bind with promoter).
The operator can be located either within the promoter or between the promoter and the structural genes.

16. It acts as a switch that turns the RNA transcription “on” or “off”.
When operator is switched “on”, the RNA polymerase binds at the promoter site and the structural genes can transcribe RNA.
When operator is switched “off”, the RNA polymerase fails to bind at the promoter and the structural genes cannot transcribe RNA.

17. Structural genes
Genes that are collectively regulated by the operon and code for functional proteins are called structural genes.
All the structural genes of an operon are turned ON or OFF together, due to a single promoter and operator upstream to them.
They code for specific enzymes involved in a metabolic pathway. Number of structural genes may vary from 3 to 5, or at times even more.

18. Two additional genes, i. e
Two additional genes, i.e., enhancer and regulator, may also plays roles in the process of gene expression.
Enhancer -- It may be present anywhere in the genome and not necessarily attached with the operon. The enhancer increases the activity of operon several times.

19. Regulator -- This gene is responsible for the secretion of regulatory proteins. Regulatory proteins are of two kinds, i.e., the activator protein and the repressor protein.
When a bacteria wants to turn on an operon, the repressor protein must be released and RNA polymerase must be bind to the promoter.
The regulatory gene does not need to be in, adjacent to, or even near the operon to control it.

20. Control of Operon 2 types of control mechanism -
Negative control operon - involves the binding of a repressor to the operator to prevent transcription.
Positive control operon - involve the binding of an activator protein usually at a site other than the operator to stimulates transcription.

21. Negative control operon
Negative inducible operons – Usually off.
Example – The lac operon
The ara operon
Negative repressible operons – Usually on.
Example – The trp operon

22. Negative inducible operon
A regulatory repressor protein is normally bound to the operator which prevents the transcription of the genes on the operon.
When the inducer is present it interacts or binds with the repressor protein, changes its conformation and releasing it from the operator. This allows for expression of the operon.

23. Lac operon
Is the classic operon example, and is responsible for the degradation of the milk protein lactose.
Is an inducible operon and here inducer is lactose.

24. Structure of lac operon
The lac operon is made up of a promoter with operator, and three genes (lacZ, lacY, and lacA) that encode enzymes for lactose degradation. The lac operon is regulated by the regulatory gene lacI. ` i
Operon
RegulatoryGene p o z y a
DNA
m-RNA
-Galactosidase
Permease
Transacetylase
Protein

28. Negative repressible operon
Repressor proteins are produced by a regulator gene, but they are unable to bind to the operator in their normal conformation. However certain molecules called co-repressors are bound by the repressor protein, causing a conformational change to the active site. The activated repressor protein binds to the operator and prevents transcription.

29. Trp operon
The Trp operon is responsible for synthesis of the amino acid tryptophan when it is not available in the environment.
Is a negative repressible operon and here the co-repressor is tryptophan.

30. Structure of trp operon
The trp operon is made up of a promoter with an operator, and five genes that encode enzymes for tryptophan synthesis. The Trp operon is regulated by the regulatory gene trpR, a gene that is located at a distance from the Trp operon. R
Operon
RegulatoryGene P O E D C
5 Proteins B A L
Inactive repressor (apo-repressor)

31. Inactive repressor (apo-repressor)D C
5 Proteins B A L
Inactive repressor (apo-repressor)
Absence of Tryptophan

32. Presence of Tryptophan Inactive repressor (apo-repressor)D C
No trp mRNA B A L
Presence of Tryptophan
Inactive repressor (apo-repressor)
Trp
(co-repressor)

34. Positive control operon
In E. coli, when glucose is always the preferred food source.
When glucose is scarce, the lac operon is activated by the binding of the catabolite activator protein (CAP).
When glucose is abundant, CAP detaches from the lac operon, which prevents RNA polymerase from binding to the promoter.

35. Promoter
Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized. If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.
(a)
CAP-binding site
Operator
RNA
polymerase
can bind and transcribe
Inactive CAP
Active CAP
cAMP
DNA
Inactive lac
repressor
lacl
lacZ
Figure 18.23a

36. lacl lacZ Promoter DNA CAP-binding site Operator RNA polymerase
Figure 18.23b
(b)
Lactose present, glucose present (cAMP level low): little lac mRNA synthesized. When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.
Inactive lac
repressor
Inactive CAP
DNA
RNA
polymerase
can’t bind
Operator
lacl
lacZ
CAP-binding site
Promoter

37. Thus, although bacteria may be considered simpler organisms than humans, it is clear that bacterial gene regulation is extremely efficient and the bacterial genome is highly organized.
Bacteria appear to be perfectly adapted to a variety of environments, and they are ready to respond to whatever environmental changes they encounter by employing elegant and complex regulatory mechanisms.

38. THANK YOU