Molecular Genetics

An AUG codon is not the only determinant of initation of protein synthesis.

Facts

• Use of the consensus sequence approach to study mRNA initiation codons revealed a preferred context for the initiating AUG. That for mammals is shown with particularly important positions indicated with arrows. Slightly different context sequences have been derived for other classes of organisms.
• Site-directed mutagenesis of sequences around the AUG confirms the conclusions from consensus analysis.
• Weak (DG= -30 kcal/mol) secondary structure close to the cap (12 nt) inhibits initiation, while the same structure 52 nt from the cap does not. More stable secondary structures inhibit scanning by the 40S initiation complex, but do not impede the progress of 80S ribosomes. Secondary structure may play a greater role than sequence context in yeast mRNA translation.
• Ribosome scanning dictates that eucaryotic mRNA's are monocistronic. Some, however, are polycistronic. Polycistrony arises in several ways:
  • The first AUG encountered may be in a poor context for translation initiation, thus allowing the ribosome to bypass the first start with some frequency. SV40 VP2 and VP3 polypeptides are made in this way.
  • The presence of IRES sequences allows cap-independent initiation.
  • By as yet ill-defined mechanisms, a ribosome after terminating a polypeptide will reinitiate at a favorable AUG, if that AUG is in close proximity to the stop codon.

Interpretations

• The binding of Met tRNA_metf via its anticodon to an AUG codon is not the sole recognition event in initiation of translation.
• Secondary structure near the cap interferes with cap recognition.
• Many exceptions to generalizations about translation initiation have been discovered in studies of expression of viral genes. Two factors contribute: study of viral gene expression has often been more intensive than that of cellular genes; viruses have evolved ways to bend host machinery to favor virus replication.

Further information

• Context sequences are also important in the incorporation of selenocysteine into proteins, the suppression of termination codons and frameshifting.
• The relative activity of IRES sequences and mutated IRES sequences can be determined using reporter genes.
• mRNAs with weak contexts for the initiating AUG are unstable because of nonsense-mediated decay (ref). Intiation at the next AUG leads to translation of a peptide to a termination codon upstream of the proper termination codon thus exposing signals for nonsense-mediated decay.

This is page 2434 of Molecular Genetics by Ulrich Melcher, © 1997, 1999