\"In most organisms, AUG and AUA triplets encode methionine and isoleucine, resp
ID: 209759 • Letter: #
Question
"In most organisms, AUG and AUA triplets encode methionine and isoleucine, respectively. This is the only case in the standard genetic code where codons that code for two different amino acids are separated only by bases of the same type at the third position. The strategy for the assignment of AUG codons to methionine, and AUA codons to isoleucine, represents a striking case of interspecies variation (Blanquet et al., 2005, Grosjean and Bjork, 2004)."
--Jones et al. (2008). Mol Cell. 29(3):401.
a. (6 points) The anticodon CAU normally would recognize both AUG and AUA. What hypothesis accounts for multiple codons recognized by a single anticodon?
b. (6 points) Studies in Mycoplasma penetrans show that a tRNA-Ile recognizes AUA and a separate tRNA-Met recognizes AUG. The tRNAs are different in the acceptor stem. Why is it necessary that methionyl-tRNA synthetase be able to distinguish between the two tRNAs?
c. (8 points) If the anticodons are the same for both tRNAs, explain how discriminator regions might help the tRNAs find the proper codon.
article link: http://www.cell.com/molecular-cell/fulltext/S1097-2765(08)00063-4?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS1097276508000634%3Fshowall%3Dtrue
Explanation / Answer
a) For multiple codon recognition by a single anticodon wobble hypothesis is refered. It explains why only 40 tRNA are needed for recognising 61 potential codons for amino acids, which is because of less specific pairing of third base of a codon.
b) This is because AUG methionine is a start codon. The AUG codon is special because it has two functions: it can either specify incorporation of methionine into the growing protein chain (elongation) or specify the starting point of the message (initiation). The discrimination between the two functions of the AUG codon is coupled to the occurrence of two structurally different methionine-carrying tRNAs (Met-tRNAs), elongator Met-tRNAm and initiator Met-tRNAi. So basically, AUG is the only start codon, and the start codon methionine is a modified special methionine, so it's absolutely imperative that there is specificity between the codon and the anticodon in this situation.
c) Mycoplasma penetrans methionyl-tRNA synthetase can directly differentiate between tRNA(Ile)(CAU) and tRNA(Met)(CAU) transcripts based on interactions with the acceptor stems of tRNA(Ile)(CAU) and tRNA(Met)(CAU). MpMRS (methionyl-tRNA synthetase) rejects tRNAIleCAU through the recognition of negative identity elements in its acceptor stem. Once methionine is bound to the proper tRNA anticodon, factors in the ribosome recognize the activated tRNA and helps it bind to the codon.
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