Several experiments were conducted to obtain information about how the eukaryoti

Question

Several experiments were conducted to obtain information about how the eukaryotic ribosome recognizes the AUG staart codon. In one experiment, the gene that encodes methionine initiator tRNA ( tRNAiMet) was located and changed. The nucleotides that specify the anticodon on tRNAiMet were mutated so that the anticodon in the tRNA was 5'- CCA- 3' instead of 5'- CAU- 3'. When this mutated gene was placed into a eukaryotic cell, protein synthesis took place but the proteins produced were abnormal. Some of the proteins produced contained extra amino acids, and others contained fewer amino acids than normal.

a. What do these results indicate about how the ribosome recognizes the starting point for translation in eukaryotic cells? Explain your reasoning.

b. If the same experiment had been conducted on bacterial cells, what results would you expect?

Explanation / Answer

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In prokaryotes, a sequence known as Shine Dalgarno sequence is present in 5' region of mRNA. This sequence is 7bp away from AUG, translational initiation codon. When this Shine Dalgarno sequence binds with the 16S rRNA, then initiation starts with the f-met-tRNA (which is the N terminal amino acid).

In Eukaryotes, such a sequence is not there. Instead the mRNA searches for the closest AUG codon in the 5' untranslated region of the sequence, 5'--CCAUGG--3'.

Now, in the given question, mutation has been done in the gene that encodes initiator tRNA. This means that some other initiator instead of methionine initiator is being used, say GUG. These new initiators have carry out the protein synthesis process. The reason for having proteins of different sizes is the difference in post translational modification of these proteins which are not recognising the methionine end at the N terminal

If the same experiment is conducted on bacteria, protein synthesis will not occur,

1. because Shine-Dalgarno sequence recognises only f-met at its 5' end, 7bp away from it.

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