Bacterial Protein Synthesis Ribosome Binding Site

For protein synthesis to take place, a ribosome must first attach to the mRNA template. AUG is not only the start codon for protein synthesis; it is used to code for all the other methionines in the protein. How does the ribosome recognize the correct AUG at which to

Figure 8.4. The P and A sites on the ribosome.

Figure 8.4. The P and A sites on the ribosome.

begin protein synthesis? All bacterial mRNAs have at their 5' end a stretch of nucleotides called the untranslated (or leader) sequence, which do not code for the protein. These nucleotides are nevertheless essential for the correct placing of the ribosome on the mRNA. Bacterial mRNA molecules usually have a nucleotide sequence similar to 5' GGAGG 3' whose center is about 8 to 13 nucleotides upstream of (5' to) the AUG start codon. This sequence is complementary to a short stretch of sequence, 3' CCUCC 5', found at the 3' end of the rRNA molecule within the 30S ribosomal subunit (Fig. 8.5). The mRNA and the rRNA interact by complementary base pairing to place the 30S ribosomal subunit in the correct position to start protein synthesis. The sequence on the mRNA molecule is called the ribosome-binding site. This is sometimes referred to as the Shine-Dalgarno sequence after the two scientists who found it.

Because the genetic code is read in triplets of three bases, there are three possible reading frames (page 79). The reading frame that is actually used by the cell is defined by the first AUG that the ribosome encounters downstream of the ribosome-binding site.

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