Mechanism of Histidine Operon

1. K. Narayanapura, Kothanur (PO), Bengaluru 560077
Tel+91 80 – 68737777 / 28465770 /28465353 Fax. 080- 68737799
e-mail:info@kristujayanti.com, www.kristujayanti.edu.in
HISTIDINE OPERON
Dr. Manikandan Kathirvel
Assistant Professor,
Department of Life Sciences,
Kristu Jayanti College (Autonomous),
Bengaluru

2.  An operon is a group of closely linked genes (structural and regulatory gene), which
regulate the metabolic pathway in prokaryotes, but not in eukaryotes.
 In eukaryotes, each gene is made up if an individual mRNA and each gene has its
own promoter.
 A bacterium contains thousands of genes, when all genes functions at a same time
the use will be flooded with enzymes and proteins. so, the genes for the required
enzyme at that particular time are switched on and the other genes are switched off.
 This ON and OFF mechanism is explained by the operon model.
Operon

3. HISTIDINE OPERON
 Histidine operon and its regulation of action in the bacterium Salmonella
typhimurium.
The bacteria controls its rate of histidine biosynthesis by 2 ways:
1. Intracellular concentration rises and feedback inhibition shuts down the
pathway
 When histidine is being transported into cell, the intracellular concentration rises
and feedback inhibition shuts down the pathway.
 When external histidine is exhausted, the histidine pool fails until feedback
inhibition is retrieved and biosynthesis of the amino acid is resumed.
2. Repression control, Governs the intracellular concentration of biosynthetic
enzymes.
This method is considerably slower in action than feedback inhibition although the
enzymes concentration may begin the new steady state level is reached.
Presence or absence of external histidine is not the only factor regulating the rate of
histidine biosynthesis.
The bacterium also keeps its rate of synthesis of amino acid in line with its growth rate.
The ability to adjust histidine production to growth rate can provide a significant economy
in metabolism.

4. Structural organization of Operon
 In Salmonella typhimurium, the enzymes responsible for the biosynthesis of histidine encoded by 9 gene
tightly clustered in a single large operon (His operon).
 Histidine operon contains a cluster of nine structural gene his G,D,C,B,H,A,F,I,E which codes for
enzyme of the pathway of the Synthesis of histidine from 5- phosphoribosyl 1- pyrophosphate (PRPP).
 Transcription produces a large single polycistronic mRNA about 7300 nucleotide long extending from a
primary promoter Hisp1 to a rho independent terminator.
 2 weak internal promoters Hisp2 and Hisp3 are located within the hisC and hisF gene respectively.
 The stop codon of each cistron overlaps the translational initiation codon of the downstream cistron.
 There are five regulatory genes his R,U,S,T and W associated with the operons, but not closely linked to it.
 The gene hisR, codes for histidine tRNA (tRNA his),while hisS codes for histidyl-tRNA synthetase.
 The histidine operon lacks operator region and a CRP site.

5. Structural organization of Operon

6. Control of Transcription
Initiation and Elongation:
 Transcription of the his operon is about four fold more efficient in bacteria growing
in minimal glucose medium that when growing in rich medium.
 This form of control called metabolic regulation adjust the expression of operon to
amino acid supply in the cell.
 It is mediated by the “alarmone” (ppGpp) which is effector of the stringent
response. (ppGpp) regulate the primary promoter Hisp1 under condition of
moderate amino acid starvation.
 The histidine operon is a cluster of a 9 gene whose expression increases in
responses to histidine starvation.
 Histidine starvation results in a 10 fold increase in the transcription of the histidine
operon. This has an inverse relationship with the amount of histidine tRNA present in
the cell.
 Deprivation of aminoacyl-tRNA directly prevents termination of transcription in
both cases, resulting in transcription of the structural genes.
Regulation: Metabolic control of histidine operon

7.  In addition to this general metabolic control, His operon transcription is specifically
regulated by attenuation of transcription a mechanism in which a regulatory element,
located upstream of the first structural gene of the cluster medulates the level of
expression of histidine biosynhetic enzymes in response to the intracellular levels of
charged histidyl transfer RNA.
 The His specific regulatory element is transcribed in a 180 nucleotide leader which
exhibits 2 promotional features:
1. 16 residue coding sequence including 7 consecutive codon specifying histidine.
2. overlapping region of dyad symmetry capable of folding into mutually exclusive
alternative secondary that signal either transcription termination or anti termination.
Regulation: Attenuation control of histidine operon

8. Attenuation control of histidine operon
Attenuation is the modification of gene expression by the events that influence aspects
of transcription and translation other than initiation of transcription .
The attenuation mechanism of tryptophan and histidine operons are similar.
The attenuator is located in the leader region of the RNA transcript that lies between the
promoter and the first structural gene. It comprises a segment coding for the leader
peptide followed by a terminator sequence.
The leader peptide coding region of the RNA transcript contains 14 codons that code
for a 14 aminoacid residue leader peptide having the following amino acid sequence :
Met-Thr-Agr-Val-Gln-Phe-Lys-His-His-His-His-His-His-His-Pro-Asp-
The leader peptide contains a sequence of seven regulatory histidine residues in a row.
In histidine operon the
concentration of histidine, the
biosynthetic end product,
regulates translation of the
leader peptide.

9. Negative regulation: RNA Termination
 Translational control of His operon transcription is determined by ribosomes occupancy of
leader RNA which in turn depends given the peculiar composition of His leader peptide on
the availability of His tRNA.
 When histidine is present in sufficient concentration, the supply of histidyl tRNA is plentiful.
 This permits the translating ribosome to pass across the leader sequence of RNA without pausing.
 The relevant codons and adjacent nucleotides are free to base pair, resulting in the formation of
the RNA terminator this prevents the translation of the his operon structural genes.
6 RNA segment are involved in
base pairing and the stem loop
structure formed by E and F
RNA region and adjacent run
of uridylated residues
constiutes the attenuation a
strong Rho independent
transcription terminator.
At high level of His t RNA, allows
rapid movement of ribosomes
upto the B segment in this case
formation of C:D and E:F stem
loop structure will result in
premature transcription
termination.
The aminoacyl tRNAs therefore functions as corepressor

10. In case of severe limitation of intracellular pH of all charged tRNAs
translation of leader peptide fails to initiate under these condition
the A:B,C:D,E:F stem loop structures from sequentially producing a
strong transcription termination.
Negative regulation: RNA Termination
 RNA polymerase pauses after synthesis of the first RNA hairpin (A:B). This pausing is believed to synchronize
transcription and translation of leader region by halting the elongation RNA polymerase until a ribosome starts
translation of leader peptide.

11. Positive regulation: Anti-termination
 When histidine is scared (low), the supply of histidyl tRNA is limited, this result
in the ribosome pausing or stalling as it passes across the leader sequence and
masking the codons, this permits the base downstream to base pair and from the
antitermination structure, which in turn prevents the formation of the terminator.
 Translation of the his operon structural genes therefore takes place .
In low level of charged t RNA-
His, ribosome delayed at the
consecutive histidine codons of
the leader peptide and prevent the
A:B pathway by making A
segment.
Base pairing between B & C and
between D & E regions prevents
formation of the attenuator and
determine the antitermination
conformation.

12.  RNA polymerase pauses after synthesis of the first RNA hairpin (A:B). This pausing is believed to synchronize
transcription and translation of leader region by halting the elongation RNA polymerase until a ribosome starts
translation of leader peptide.

13. Thank You