ATP requirement for binding of 125I-labeled globin mRNA to Artemia salina ribosomes

Initiation factors eIF4A and C1 from wheat germ and the formation of mRNA X ribosome complexes

The binding of ribosomes to mRNA is analyzed in a fractionated system from wheat germ with [3H]uridine-labeled poly(A)+ RNA prepared from germinating wheat embryos. The reaction requires factors eIF3, eIF4C, and eIF5; Met-tRNA and the Met-tRNA binding system; either GTP or GMP-PNP; ATP; and factors C1 and eIF4A. These requirements are identical to those previously found to be necessary for formation of ribosome X Met-tRNAMeti complexes, with the exception of ATP, and factors C1 and eIF4A. The function of factors C1 and eIF4A is therefore specifically related to the mRNA attachment reaction. The presence of GTP in the mRNA binding reaction results in the formation of 80 S ribosome complexes, while with GMP-PNP only 40 S ribosome complexes are formed. Ribosome binding to native reovirus RNA in the fractionated wheat germ system is similar to the reaction with poly(A)+ RNA, strongly requiring ATP and factors C1 and eIF4A. Binding to inosine-substituted reovirus RNA, however, is only partially dependent upon ATP, and both the ATP-dependent and the ATP-independent binding reactions strongly require factor C1 and are substantially stimulated by factor eIF4A. The ATP-independent reaction is inhibited by pm7GDP, has a strong requirement for Met-tRNAMeti, and the 40 S ribosome complex is stable to RNase. These results indicate that the ATP-independent binding of ribosomes to inosine-substituted reovirus RNA proceeds through the normal initiation process. They further suggest that neither factor C1 nor eIF4A function exclusively to unwind mRNA secondary structure. Since eIF4A is required for the ATP-independent binding to inosine mRNA, and at the same time interacts with ATP in the reaction with ATP-requiring mRNAs, this factor may have two roles in protein chain initiation, one related to the mRNA X ribosome interaction, and one related to the function of ATP.

Increased ribosomal affinity for mRNA causes resistance to edeine in a mutant of Saccharomyces cerevisiae

The effect of edeine on the translation of mRNA or poly(U)-directed polyphenylalanine synthesis has been studied in an edeine-resistant mutant of Saccharomyces cerevisiae under three different experimental conditions: in the whole lysate system, in a micrococcal-nuclease-treated lysate, and in a high-salt-treated lysate. The results indicate that translation of messenger is more resistant to edeine in the whole lysate than in the depleted lysates; these observations suggest that resistance to edeine is associated with the presence of endogenous mRNA. It is shown that 40S mutant subunits have a higher affinity for polysomal RNA than 40S wild-type subunits. Since the mRNA binding is inhibited by 7-methylguanosine 5'-monophosphate, the interaction between polysomal RNA and 40S ribosomes is specific for mRNA. The data demonstrate that in each of the depleted lysates, with edeine initially present, the formation of the 80S initiation complex is inhibited. However, edeine inhibition of [3H]methionine binding to 80S ribosomes is overcome completely in the mutant extract by preincubation of this lysate with polysomal RNA. The results indicate that the mutant may carry a specific change in a messenger-binding factor or in a ribosomal protein thereby permitting an increased stability of the messenger-ribosome complex which consequently results in an increased resistance of the mutant lysate to edeine.

An edeine resistant mRNA-dependent protein synthesis system from a Saccharomyces cerevisiae mutant

A cell-free protein synthesizing system from a mutant of Saccharomyces cerevisiae translated exogenous mRNA in the presence of 2 microM edeine, while a similar system from wild-type strain was completely inhibited by the drug. The mutant ribosomes showed an affinity for [125I]edeine comparable to the wild-type ribosomes, thereby suggesting that these macromolecules alone were not responsible for the edeine-resistant capacity of the mutant.

Role of ATP in binding and migration of 40S ribosomal subunits

Two assays have been devised to demonstrate ATP-dependent migration of 40S ribosomal subunits on messenger RNA. The first is a two-step runoff assay. Reovirus mRNA was initially loaded with 40S subunits by incubation with wheat germ ribosomes in the presence of the antibiotic edeine. During the second phase of the incubation, in which further attachment of ribosomes was inhibited, the preformed complexes were shown to dissociate (presumably by runoff) only if ATP was included in the reaction. A more direct demonstration of ATP-dependent migration of 40S subunits was carried out using 3' end-labeled brome mosaic virus mRNA. In the presence of edeine and ATP, 40S ribosomal subunits were shown to advance all the way to the 3' end of the message, as shown by protection of the labeled 3'-proximal segment against nuclease digestion. Depletion of ATP by the addition of hexokinase prevented this migration. A variety of observations has raised the possibility that attachment of eucaryotic ribosomes to messenger RNA proceeds via a "scanning mechanism." The hypothesis is that a 40S subunit binds initially at or near the 5' terminus of the message and subsequently migrates toward the interior, stopping when it encounters the first AUG triplet. If migration of 40S subunits requires ATP, as the present studies suggest, the scanning mechanism predicts that in a system depleted of ATP a single 40S ribosome should be trapped near the 5' terminus of the message--upstream of the AUG initiator codon. This prediction was confirmed by analyzing binding of wheat germ ribosomes to a synthetic ribopolymer in which the 5'-proximal region (lacking AUG codons) and the AUG-containing segment near the 3' end of the molecule were differentially labeled.