researchers have isolated strains of bacteria that carry mut

C4. Researchers have isolated strains of bacteria that carry mutations within tRNA genes. These mutations can change the sequence of the anticodon. For example, a normal tRNAmp gene encodes a tRNA with the anticodon 3\'-ACC-5\'. A mutation could change this sequence to 3\'-CCC-5\'. When this mutation occurs, the tRNA still carries a tryptophan at its 3\' acceptor stem, even though the anticodon sequence has been altered. A. How would this mutation affect the synthesis of polypeptides within the bacterium? B. What does this mutation tell you about the recognition between tryptophanyl-tRNA synthetase and tRNATmp? Does the enzyme primarily recognize the anticodon or not?

Solution

The following diagram depicts the changes in the tRNA anticodon by mutation and the protein translation frame of amino acids after mutation in tryptophan---

A) After mutation of anticodon sequence -3\'-ACC-5\'- in normal tRNA tryptophan  to --3\' CCC-5\' in mutant tryptophan tRNA , the mutant enzyme still encodes for the tryptophan amino acid due to wobble effect of translation which does not effect the addition of tryptophan on the polypeptide synthesized by the bacteria. Various chimeric studies have been performed with changes in the DHU loop,T phi-C loop ,and the anticodon loop of the tRNA but it does not effect the addition of the same amino acid on the polypeptide from the acceptor stem of the tRNA, however a change in the acceptor stem nucleotide by mutation alters the amino acid attached on the acceptor-5\' stem of the tRNA with acyl tRNA synthetase and therefore a different amino acid gets attached on the polypeptide chain. The mutation in the anticodon of tRNA with amino acyl tRNA synthetase sterically reorients the attached tryptophan on the acceptor stem of the enzyme but still the same amino acid is attached on the polypeptide.

B-The nucleotides on proline tRNA anticodon loop  and tRNA tryptophan anticodon loop have been mutated accordimgly and studies have been done to analyse the methyl transferase activity of these enzymes and the amino acids attached on the acceptor stem of the tRNA which add methyl groups on guanine nucleotides. The reognition of trytophanyl tRNA synthetase and tRNA tryptophan gene is the same after mutation also in the anticodon loop.