Molecular Genetics

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In procaryotes, the signal for initiation of protein synthesis consists primarily, but not exclusively, of an AUG codon and a rRNA-complementary sequence, the Shine Dalgarno sequence.

Facts

• Procaryotic mRNAs are often polycistronic, containing several translation units, or cistrons,on the same mRNA. Positions of cistrons define non-cistronic regions on the mRNA. Arrows point to the 5' untranslated region (a), the intercistronic region (b), and the 3' untranslated regions (c). Polycistronic mRNAs include those from lac zya and trpEDBCA genes. Examples of monocistronic mRNAs are those from lacI and trpR genes. Some genes can be transcribed in either monocistronic or polycistronic forms (lambda N et al.; lambda cro et al).
• Protein synthesis is initiated at an AUG codon using a formylated methionyl tRNA that only acts at initiation.
• An "AGGAGG ", (Shine Dalgarno), sequence is 4-7 nucleotides 5' of the initiator AUG of many mRNAs. The sequence is complementary to gaucaCCUCCUuaOH at the 3' end of 16S rRNA.
• Site-specific mutation of either the mRNA or 16S rRNA sequence motifs to decrease complementarity reduces the efficiency of initiation of translation.
• Fragments containing only a Shine-Dalgarno sequence and an AUG serve as good initiators if they derive from actual protein synthesis initiation sites.

Interpretations

• The requirement for a Shine-Dalgarno sequence in addition to AUG for proper initiation allows the AUG to be chosen from among multiple AUG trinucleotides in mRNA, most coding for internal methionines or representing out of phase codons.
• Binding of mRNA to rRNA via the Shine Dalgarno sequence may stimulate initiation by increasing the local concentration of AUG near the correct site on the ribosome.
• Other sequences, in addition to the AUG and Shine-Dalgarno sequence, are also important.

Further information

• RNA secondary structure may also play a role in the efficiency of use of particular AUG codons in initiation. Accessibility appears to have been used by bacteriophage MS2 in allowing its maturation gene to be expressed transiently while the RNA is folding (ref).
• Details of initiation of protein synthesis in eucaryotes differ from these.

This is page 2431 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 1999