What is the Shine–Dalgarno sequence?
According to this Wikipedia article:  


  "The Shine-Dalgarno (SD) sequence is a ribosomal binding site in
  bacterial and archaeal messenger RNA, generally located around 8 bases
  upstream of the start codon AUG.1 The RNA sequence helps recruit the
  ribosome to the messenger RNA (mRNA) to initiate protein synthesis by
  aligning the ribosome with the start codon.
  
  The Shine-Dalgarno sequence exists both in bacteria and archaea. It is
  also present in some chloroplast and mitochondrial transcripts. The
  six-base consensus sequence is AGGAGG; in Escherichia coli, for
  example, the sequence is AGGAGGU..."  


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