Discussion
In the prokaryotic protein synthesis, initiation site of mRNA consists of..............codon preceeded by ..........
Answer: A
The initiation site is the AUG codon preceded by the Shine-Dalgarno polypurine sequence, the paper's 'Shine-Dalgamo polypurine hexamer'.
In bacteria the start codon, usually AUG, lies a few bases downstream of a purine-rich stretch (consensus AGGAGG) that pairs with the 3' end of the 16S rRNA. That pairing places the start codon in the P site of the small ribosomal subunit, so the right AUG is chosen even in the middle of a polycistronic mRNA.
The 5' cap and its 7-methylguanosine (m7G), which appear in the other options, belong to eukaryotic mRNA, where the small subunit binds the cap and scans for the first AUG. UAG is a stop codon, and AGG codes for arginine; neither is a start codon. GUG can occasionally start bacterial genes, but it is not paired with a cap.
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