Search

Tobacco mosaic virus and turnip yellow mosaic virus genome RNAs, which have long (greater than or equal to 69 nucleotides) 5'-terminal leader sequences preceding the AUG initiation codon, can form disome initiation complexes when incubated with a wheat germ extract and sparsomycin. In tobacco mosaic virus RNA, whereas one ribosome in such complexes presumably occupies the AUG-containing initiation site, the other is located within the RNA leader sequence. The results favor a model of eukaryotic initiation in which ribosomes first bind to the 5' end of the mRNA independently of AUG, then advance to the AUG initiation codon, where protein synthesis ensues.

In amino acid polymerization, the tRNA donating its peptidyl moiety to the neighboring aminoacyl-tRNA must be released from the ribosome for further growth of the polypeptide chain. It was not known at what stage of peptide elongation this tRNA is released. To study this question, we prepared ac-C 14 -Phe-H 3 -tRNA using H 3 -tRNA isolated from E. coli strain 15THU grown in the presence of H 3 -uracil, and followed the fate of the H 3 -tRNA during chain growth. Our results indicate that donor tRNA is released during the translocation step, mediated by the soluble factor G and GTP.

The 3′ terminal nucleotide of turnip yellow mosaic virus (TYMV) RNA (23-25 S) may be esterified with valine in the presence of ATP and an enzyme preparation from Escherichia coli . The nucleotide composition near the valine-binding site is different for TYMV RNA and tRNA Val from cabbage, as shown by comparison of the valine adducts of nucleotides labeled with radioactive valine in T 1 RNase digests. Consequently, host tRNA Val is not involved in the observed charging of TYMV RNA with valine. The TYMV RNA appears to have a tRNA-like structure, at or near its 3′ end, that is recognized by three different enzymes which specifically catalyze reactions involving tRNA.