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Wednesday, January 09, 2008

The sigma (s) subunit of prokaryotic RNA Polymerase Specifically recognizes the promoter site

043. The sigma (s) subunit of prokaryotic RNA Polymerase:

1. Binds the antibiotic Rifampicin

2. Is inhibited by a-amanitin

3. Specifically recognizes the promoter site

4. Is part of the core enzyme

Answer

3. Specifically recognizes the promoter site

Reference:

Harper 26th Edition Page 343

Lippincott 3rd Edition Page 415

Chaterjee 6th Edition Page 233

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Discussion

Transcription by prokaryotic RNA Polymerase involves a core enzyme and several auxiliary proteins

1. Core Enzyme : Four of the enzymes subunits, 2 a, 1 b and 1b’ are responsible for the 5’-3’ RNA polymerase activity and are referred to as the core enzyme. However this enzyme lacks specificity, that is, ti cannot recognize the promoter region on the DNA template

2. Holoenzyme : The sigma (s) subunit (Sigma factor) enables RNA polymerase to recognize promoter regions of the DNA. The sigma (s) subunit plus the core enzyme makes up the holoenzyme. Different sigma (s) factors recognize different groups of genes

3. Termination factor : Some regions on the DNA that signal the termination of transcription are recognized by the RNA polymerase itself. Others are recognized by specific termination factors, an example of which is the rho (r) factor of E.coli

Explanation

1. Rifampicin and Streptovaricin bind with the beta subunit of RNA Polymerase to block the initiation of trascription

2. RNA Polymerase II (of eukaryotic RNA synthesis) is inhibited by a-amanitin

3. With the assistance of sigma (s) factor, the core enzyme recognizes a promoter

4. The (s) factor is not a part of the core enzyme

Comments

Ä Action of Drugs :

o Actinomycin D forms a complex with double stranded DNA and prevents the movement of core enzyme and as a result inhibit the process of chain elongation

o Streptoglydigin bnds with the beta subunit of prokaryotic polymerase and this inhibits elongation

o Heparin binds to the beta’ subunit

Ä Eukaryotic Gene Transcription

o Nuclear RNA polymerases

§ RNA Polymerase I: synthesizes the precursor of the large ribosomal RNAs(28S, 18S and 5.8S) in the nucleolus. Please note that mRNA and tRNA are synthesized in the nucleoplasm.

§ RNA Polymerase II: synthesizes the precursor of messenger RNA that are subsequently translated to produce proteins. Polymerase II also synthesizes certain small nuclear RNAs (snRNA) and is used by some viruses to produce viral RNA

§ RNA Polymerase III: produces small RNAs including small tRNAs, the small 5S ribosomal RNA and some snRNA

o Mitochondrial RNA Polymerases

§ Mitochondria contain a single RNA polymerase that resembles bacterial RNA polymerase more closely that it does the eukaryotic enzyme

Tips

Do you know that the following medical terminologies originated from Greek Alphabets

Ä Delta (D d) like

o Deltoid(Muscle),

o Delta Peptide (Sleep Inducing Peptide)

o Delta Rhythm (EEG),

o Delta Virus (Hepatitis D Virus),

Ä Lambda (L l) like

o Lambdoid (Suture Defect),

o Lambda Phage (Bacteriophage)

o Lambda Sign (Small Ascending Aorta)

Ä Sigma (S s) like

o Sigmoid (Colon)

o Sigma Factor (86 kD-binds RNA Polymerase.)

Ä Upsilon (u Y) like

o Hyoid (Bone)

Ä Chiazo (c C) like

o Chiasma(in Chromosomes)

o Optic Chiasma

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