Protein synthesis in rabbit reticulocytes: factors controlling terminal and internal methionine codon (AUG) recognition by methionyl tRNA species

The efficiency of methionine incorporation from isoaccepting species of tRNAMet into rabbit globin in an homologous reticulocyte lysate system

Rabbit globin alpha and beta chains were labeled with [3H]leucine, and with [35S] -methionine from reticulocyte tRNAMet isoacceptors using a rabbit reticulocyte cell-free synthesis system. [35S]Methionine from the three tRNAMet species isolated by RPC-5 chromatography was incorporated into internal positions of both alpha and beta globin. The initiator tRNA, tRNAIMet, exhibited very low efficiency for incorporating methionine internally, while tRNAIIMet was four times more efficient than tRNAIIIMet. Amino acid analysis of the tryptic peptides of the labeled globins revealed that all three isoacceptors incorporated methionine into the normal methionine peptides. Similar studies with Escherichia coli [35S]Met-tRNAfMet showed a 3-fold increase over the reticulocyte initiator tRNA in its capacity to incorporate methionine into the internal positions of rabbit globin.

Absence of the sequence G-T-psi-C-G(A)- in several eukaryotic cytoplasmic initiator transfer RNAs

The nucleotide sequence G-T-Psi-C-G(A)- has previously been found in every tRNA of known sequence that is active in protein biosynthesis. An exception to this generalization is the recently sequenced initiator tRNA from yeast cytoplasm. It is now reported that cytoplasmic initiator tRNAs from wheat germ, rabbit liver, and sheep mammary gland also lack the G-T-Psi-C-G(A)- sequence. Thus: (i) nucleoside composition analyses show the absence of T in all these tRNAs; (ii) analyses of oligonucleotide fragments produced by T1 ribonuclease show the absence not only of the T-Psi-C-G(A)- sequence, but also of U-Psi-C-G(A)- or U-U-C-G(A)- sequences in such digests. The absence of G-T-Psi-C-G(A)- in the eukaryotic cytoplasmic initator tRNAs is, therefore, not simply due to lack of enzymatic modification of U to T.

Initiation of picornavirus protein synthesis in ascites cell extracts

The current model of picornavirus protein formation implies that initiation of protein synthesis occurs at a single site on the viral RNA, and that the large polypeptide formed is later cleaved. A direct test of this model was made in vitro by studying the incorporation of [(35)S]methionine from rabbit liver Met-tRNA(M) (Met) and fMet-tRNA(F) (Met) into encephalomyocarditis virus RNA-coded proteins in extracts of Ehrlich ascites cells. The incorporation of N-formylmethionine was complete within 5 min, while utilization of Met-tRNA(M) (Met) continued for 20 min. Tryptic digests of [(35)S]methionine-labeled products from Met-tRNA(M) (Met) analyzed by anion-exchange chromatography yielded more than 30 peptides, as compared to about 15 [(35)S]methionine-labeled peptides from purified encephalomyocarditis virus. In contrast, products labeled with fMet-tRNA(F) (Met) yielded one major (26)S-labeled tryptic peptide. The N-terminal location of methionine in this peptide was verified by Edman degradation. One predominant N-terminal tryptic peptide was also obtained with fMet-tRNA(F) (Met) when mouse Elberfeld and mengo-virus RNAs were used as messengers. On the basis of N-terminal compared with internal labeling of the products, no evidence for in vitro post-translational cleavage was found. The results are consistent with a single initiation site for synthesis of picornavirus proteins.

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.