Initiation of protein synthesis: evidence for messenger RNA-independent binding of methionyl-transfer RNA to the 40 S ribosomal subunit

Inhibitory effects of 'cap' analogues on globin mRNA and encephalomyocarditis RNA translation in a reticulocyte cell-free system

The cap analogues 7-methylguanosine 5'-phosphate [m7G(5')p], 7-methylguanosine 5'-triphosphate [m7G(5')ppp] and 2'-O-methylguanosine 5'-triphosphate [Gm(5')ppp] inhibit the translation of capped globin mRNA and encephalomyocarditis (EMC) RNA (a naturally uncapped mRNA) in a reticulocyte cell-free system. This inhibition occurs at the level of protein synthesis initiation and is of a competitive type since it can be overcome by increasing the mRNA concentration. However, the translation of globin mRNA is more sensitive to the inhibitory effects of the cap analogues m7G(5')p and m7G(5')ppp than translation of EMC RNA. The same spectra of specific inhibition is also observed with some other initiation inhibitors such as aurintricarboxylic acid, which inhibits mRNA binding, but not with pactamycin which does not affect mRNA interaction. A model is presented suggesting that this preferential inhibition by cap analogues could be explained mainly by the different affinities of globin mRNA and EMC RNA for the initiation complexes between 40-S subunits and Met-tRNAf. Moreover Gm(5')ppp cannot be considered simply as a cap analogue since it also affects some step prior to mRNA binding.

The coupling of transcription from influenza virions to translation in vitro

The optimum conditions for the coupling of fowl plague virus (FPV) transcription to an in vitro reticulocyte translation system have been established and shown to be close to those required for maximum RNA synthesis by purified FPV virions. Products have been characterized by the peptides they yield on limited proteolysis in SDS and it has been shown that virus nucleoprotein (NP) and matrix (M) protein are made. The smallest virus coded polypeptide, the non-structural protein (NS), is made in only small amounts in the coupled system although it is a major virus coded product of infected cells early in infection.

Regulation of protein synthesis in mitotic HeLa cells

Mitotic HeLa cells (M cells) synthesize protein at about 25% of the rate of S phase cells. This decrease in protein synthesis is due to a reduction in the rate of initiation. However, extracts prepared from M cells are almost as active in protein synthesis as S cell extracts. Both cell extracts are quite active in in vitro initiation of protein synthesis. Moreover, two steps in initiation, binding of Met-tRNAf to 40S ribosomal subunits and binding of mRNA to ribosomes, show similar activity in both extracts. The difference in protein synthesizing activity observed in vivo is largely eliminated in the preparation of cell-free systems. The ribosomes of M cells contain small mol wt RNA, which inhibits protein synthesis in vitro. This RNA, which has possibly a nuclear origin, may be a cause of the reduction in the rate of protein synthesis in M cells.

The mode of action of alpha sarcin and a novel assay of the puromycin reaction

A new technique was developed for measuring the amount of peptidyl-tRNA in a protein-synthesizing system in vitro. By this technique the course of the puromycin reaction may be followed and the modes of action of various inhibitors of protein synthesis readily determined. We conclude that the polypeptide alpha sarcin inhibits the binding of aminoacyl-tRNA into the ribosomal 'A' site, that sparsomycin inhibits the peptidyl transferase reaction and that cycloheximide may block translocation.

Characteristics of a coupled cell-free transcription and translation system directed by vaccinia cores

1. A coupled transcription and translation system is described in which protein synthesis is directed by mRNA synthesised in situ by vaccinia virus cores. The cell-free system is based on a micrococcal-nuclease-treated reticulocyte lysate. 2. The polypeptides made in vitro include many authentic early vaccinia proteins, but also other proteins which were not detected in infected cells. 3. Concentrations of cores which inhibit host cell protein synthesis in vivo caused a delayed inhibition of translation in vitro; this was partly, but not entirely, due to dsRNA associated with the cores. 4. The mRNA made was methylated by core enzymes. Inhibition of methylation reduced the rate of translation tenfold; unmethylated RNA bound ribosomes poorly, but was nevertheless translated faithfully.

Phosphorylation of initiation factor elF-2 and the control of reticulocyte protein synthesis

When rabbit reticulocyte lysates are incubated in the absence of hemin or in the presence of low concentrations of double-stranded RNA, the rate of initiation of protein synthesis is severely reduced after a lag period in which control rates are observed. This reduced initiation rate is due to inhibition of the binding of Methionyl-tRNAf to native 40S ribosomal subunits and is caused by a macromolecular inhibitor which is activated under these conditions. This paper shows that the inhibitors activated in these two situations appear to be different entities, but that in both cases, the inhibitor has an associated protein kinase activity which is highly selective for the small subunit of elF-2, the initiation factor which catalyzes binding of Methionyl-tRNAf to 40S subunits. We present several lines of evidence in support of the hypothesis that the phosphorylation of elF-2 by these kinases is basis of the control of initiation in lysates incubated under these conditions.

Resolution of ribonucleic acids by Sepharose 4B column chromatography

Ribonucleic acids were resolved by molecular sieve chromatography on columns of Sepharose 4B. The elution positions of messenger ribonucleic acids were determined by detection of polyadenosine tracts and by support of protein synthesis in a messenger-dependent cell-free system. The elution position of other ribonucleic acid species from the Sepharose 4B was determined by formamide-sucrose density gradient centrifugation. Resolution of ribonucleic acids by this column was not dependent on molecular weight but rather on other properties such as secondary structure or the presence of poly(adenylic acid). The elution profiles of ribonucleic acids on cross-linked Sepharose 4B differed markely from those on conventional Sepharose and appeared to depend on molecular size alone. There was diminished resolution of high molecular weight ribonucleic acids on such columns.

Monocistronic translation of alfalfa mosaic virus RNAs

The four alfalfa mosaic virus RNAs (respectively 24 S, 20 S, 17 S and 12 S) have been used separately as messengers in two in vitro protein synthesizing systems: wheat germ and rabbit reticulocyte lysate. In both systems a polypeptide corresponding to the translation of the entire length of the RNA can be found for RNAs 24 S, 20 S and 12 S, but not for 17 S RNA, the translation product of which is only 35,000 daltons. The number of initiation sites has been determined for each RNA by analyzing the initiation peptides synthesized in the presence of spasomycin and show that there is only one initiation or binding site perRNA. We thus conclude that each AMV RNA behaves as a monocistronic messenger in in vitro translating systems.