AUG initiation codons-eukaryotes

In eucaryotes, the signal for initiation of protein synthesis consists primarily, but not exclusively, of an AUG codon and the 5' end of the mRNA.

Subcellular fractions of cultured mammalian cells, when supplied with a source of energy, a mRNA, and amino acids, will incorporate those amino acids into protein. The positive sense virion RNA of encephalomyocarditis virus (EMC) is a convenient mRNA for such experiments. It acts as template for protein synthesis at both low and high magneisum ion concentrations.
The synthetic RNA, polyuridylic acid, leads to phenylalanine incorporation into protein only at high magnesium ion concentration.
Addition of an AUG trinucleotide to the 5' end of polyuridylic acid results in an RNA that programs polyphenylalanine synthesis at both high and low magnesium ion concentrations.
Two distinct methionyl tRNAs were isolated from the cultured cells, despite the existence of but a single codon for methionine in the genetic code.
Each tRNA was separately charged with radioactive methionine. One (tRNA _meti) was inactive in donating methionine to the polypeptide programmed by AUG(U)_n, while the other (tRNA _metf) was active.
With EMC RNA as template, greater methionine incorporation was observed from tRNA _meti than from tRNA _metf.
Met tRNA_metf was formylatable by E. coli enzymes.

Comparisons of mRNA nucleotide sequences with the amino acid sequences of their translation products revealed that, in 90 of 95 cases, the 5'-most AUG is the first codon translated. The first AUG is 10-300 nucleotides from the 5' end (most commonly 40 to 80 nucleotides). No features such as complementarity to the 3' end of the small rRNA are evident, though the immediate context of the AUG is non-random.

Natural mRNAs (like EMC RNA) have a signal that allows their translation at low magnesium ion concentration. That signal is mimicked by the addition of a methionine codon to a synthetic RNA, suggesting that AUG is a signal for initiation of translation.
The finding of a special tRNA that only donates its methionine residue to the beginning of polypeptides supports a signal role for the AUG codon in translation initiation.
That AUG is the principal initiation signal can be concluded from the nucleotide-amino acid sequence comparison. Rarely, other codons also function as initiators.
Initiation of translation in eucaryotes is similar to that in procaryotes in that the signal is an AUG codon, that a special tRNAmet is required for initiation, and that the met tRNA is formylatable. In contrast to initiation in E. coli, no eucaryotic formylase exists and complementarity of eucaryotic mRNAs to 3' ends of 18S rRNA is not important.
Mutations in the initiating AUG are an absolute loss of signal. Translation may start at the next AUG (eukaryotes only), which may or may not be in phase. If another trinucleotide mutates to AUG, and it is 5' to the wild-type initiating AUG, a new start will result if the context is OK.

In Archaea, though 70% of the open reading frames begin with AUG, 25% appear to start with GUG and 5% with UUG.
The cap structure is the feature of the 5' end of mRNA important for initiation of protein synthesis.

E-mail inquiries to U. Melcher------------Last Updated: 9 November, 1999

Polyphenylalanine Synthesis
	at [Mg++]
	2 mM	10mM
EMC RNA	+	+
poly U	-	+
AUG(U)_n	+	+