Several experiments were conducted to obtain information about how the eukaryotic ribosome recognizes the AUG start codon. In one experiment, the gene that codes for 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.
a. What do these results indicate about how the ribosome recognizes the starting point
for translation in eukaryotic cells? Explain your reasoning.
By mutating the anticodon to 5'–CCA–3' from 5'–CAU–3' on tRNAiMet, the initiator tRNA will now recognize the codon 5'–UGG–3', which normally would code only for Trp. If translation initiation by the ribosome in eukaroytes occurs by binding the 5' cap of the mRNA followed by scanning, then the first 5'–UGG–3' codon recognized by the mutated tRNAiMet will be the start site for translation. If the first 5'–UGG–3' codon occurs prior to the normal 5'–AUG–3' codon, then a protein containing extra amino acids could be produced. If the first 5'–UGG–3' codon occurs after the normal 5'–AUG–3', then a shorter protein will be produced. Finally, truncated proteins could also be produced by the first 5'–UGG–3' being out of frame of the normal coding sequence. If this happens, then most likely a stop codon will be encountered before the end of the normal coding sequence and will terminate translation. The data suggest that translation initiation takes place by scanning of the ribosome for the appropriate start sequence.