Peter John M.Phil, PhD Atta-ur-Rahman School of Applied Biosciences (ASAB) National University of Sciences & Technology (NUST)

Using The Genetic Code

Using the Genetic Code-1 Wobble Base paring Structure of tRNA Codon Anticodon Pairing Modified Bases

Using the Genetic Code-2 tRNA Charging Aminoacyle synthetases Structure, function of synthetases Proof Reading Mechanism

Using the Genetic Code-3 Suppressor tRNAs Mechanism of Suppression Frame Shifting Programmed Frame Shifting

Suppressor mutation A suppressor is a second mutation that restores a function lost by the primary mutation. A suppressor mutation that occurs within the same gene is called an "intragenic suppressor", and a suppressor mutation that occurs in a different gene is called an "intergenic suppressor"

Suppressor mutation A mutation that overcome the affect of other mutation is called “suppressor” Suppressor tRNA restore the function of mutant protein, it may be non-sense/missense In wild type cell mutation is recognized by release factor which terminate protein synthesis. If different AA is inserted than the original one then active protein is alerted

Suppressor tRNAs A suppressor tRNA typically has a mutation in the anticodon that changes the codons to which it responds. When the new anticodon corresponds to a termination codon, an amino acid is inserted and the polypeptide chain is extended beyond the termination codon. This results in nonsense suppression at a site of nonsense mutation or in read through at a natural termination codon.

Stop Codon Suppressor Amber Suppressor (UGA) Ochre Suppressor (UAA, UAG)

Suppressor Compete With Wild Type Non Sense suppressor tRNA compete with release factor that recognize stop codon A missense suppressor tRNA compete with tRNA that respond properly to its new codon The effectiveness of a suppressor depends upon its affinity & its concentration in the cell

Suppressor Compete With Wild Type Non sense suppressor may also suppress stop codon causing read through & elongation of protein, which can be deleterious Amber suppressor (UAG) are active (10-50%) depending upon the environment Ochre suppressor (UAA, UAG) below 10% UAG is misread by Trp-tRNA (1-3%)

Suppressor tRNA Suppressor tRNA have 2 consequence (i) It recognize new codon (ii) some time prevent tRNA to respond

Mechanism of suppression

Ribosome Influence Translation Accuracy Two stages when error can occur in protein synthesis (i) Charging a tRNA with correct AA (ii) Codon/anti codon binding Ribosome & tRNA also influence protein binding Codon & anticodon

Ribosome Influence Translation Accuracy Two model that ribosome discriminate b/w correct & incorrect tRNA Direct recognition model: in proper/normal binding conformational change occur in aminoacyle tRNA, which ribosome can recognize Kinetic Proof Reading Model: An incorrect aminoacyle tRNA can pas through some stages before its degradation

Ribosome Influence Translation Accuracy

change in codon meaning Suppression involve recognition of a codon by mutant tRNA that usually respond to different codon Re definition of the meaning of a codon occurs when an aminoacyle tRNA is modified. The reading frame can be changed by framshifting or bypassing, both of which depending upon the properties of the mRNA.

Frame shifting Frame shifting occur in 2 ways (i) some mutant tRNA recognize 4 bp codon instead of 3 bp (ii) slippery seq allow tRNA to move base up/down mRNA in the A site

Frame shifting

Programmed Frame shifting Some times frame shifting is important for the expression of certain genes, this is called programmed frame shifting Delay in ribosome can also cause frame shifting

By Passing By Pass can occur due to (i) Ribosomal delay (ii) Starvation in Amino Acid ribosome miss one stop codon & termination occur at other, when identical (or synonymous) codons at either end of the sequence that is skipped. They are sometimes referred to as the "take-off" and "landing" sites.

By Passing

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