A possible role of the AUG codon in the initiation of polypeptide synthesis in a eukaryotic orgamism

Sequence homology at the 5'-termini of insect messenger RNAs

Treatment of insect polyribosomes with 1 M KCl released a messenger ribonucleoprotein with a pronounced 16S peak. Phenol extraction resulted in a defined peak of 10S RNA, which was judged as mRNA by the following criteria: it showed specificity for binding to ribosomes, and the formation of initiation complex was dependent on protein initiation factors, GTP, mRNA, and aminoacyl-tRNA. The complex directed protein synthesis upon the addition of elongation factors. mRNA was treated with phosphatase and phosphorylated at the 5'-end with [(32)P]cyanoethylphosphate. [(32)P]mRNA was digested by T1 ribonuclease to completion and chromatographed on DEAE-cellulose. The only fragment with (32)P was 15 nucleotides long; it was treated with pancreatic ribonuclease and fingerprinted. Fractions of AC, AAC, and AAAC were found. Initiation signal AUG or GUG in these mRNAs does not begin immediately at the 5'-end and may be at a distance greater than 15 nucleotides. Alkaline hydrolysis of mRNAs labeled in vivo with [(14)C]adenosine revealed Ap and pppAp. Alkaline hydrolysis of mRNA labeled with (32)P at the 5'-terminus resulted in pAp. Hence, these results suggest that in a heterogeneous population of mRNAs from insects, all start with A and have sequence homology at the 5'-termini. This sequence may reflect the signal for RNA polymerase on the gene or may promote the binding of mRNA to ribosomes.

Initiation of protein synthesis in HeLa cells

Initiation of protein synthesis in HeLa cells has been synchronized by exposure of the cells to fluoride. Double-labeling of such cells for short pulses with [(35)S]methionine and a tritiated amino acid, followed by Edman degradation of the puromycin-released nascent peptides, has shown that the percent of N-terminal methionine incorporated compared to total incorporation is significantly higher than the value obtained with any of the other amino acids tested. The results suggest that the bulk of the nascent proteins synthesized in vivo by HeLa cells are initiated with methionine.