Preparation and analysis of L-( 35 S)methionine labeled transfer ribonucleic acids from rabbit liver

New chromatographic and biochemical strategies for quick preparative isolation of tRNA

A combination of hydrophobic chromatography on phenyl-Sepharose and reversed phase HPLC was used to purify individual tRNAs with high specific activity. The efficiency of chromatographic separation was enhanced by biochemical manipulations of the tRNA molecule, such as aminoacylation, formylation of the aminoacyl moiety and enzymatic deacylation. Optimal combinations are presented for three different cases. (i) tRNA(Phe) from Escherichia coli. This species was isolated by a combination of low pressure phenyl-Sepharose hydrophobic chromatography with RP-HPLC. (ii) tRNA(Ile) from E. coli: Aminoacylation increases the retention time for this tRNA in RP-HPLC. The recovered acylated intermediate is deacylated by reversion of the aminoacylation reaction and submitted to a second RP-HPLC run, in which deacylated tRNA(Ile) is recovered with high specific activity. (iii) tRNA(i)(Met) from Saccharomyces cerevisiae. The aminoacylated form of this tRNA is unstable. To increase stability, the aminoacylated form was formylated using E.coli: enzymes and, after one RP-HPLC step, the formylated derivative was deacylated using peptidyl-tRNA hydrolase from E.COLI: The tRNA(i)(Met) recovered after a second RP-HPLC run exhibited electrophoretic homogeneity and high specific activity upon aminoacylation. These combinations of chromatographic separation and biochemical modification can be readily adapted to the large-scale isolation of any particular tRNA.

Interaction of wheat germ translation initiation factor 2 with GDP and GTP

The wheat germ translation initiation factor 2 (WGeIF-2) was isolated in a homogeneous state by an efficient procedure and characterized. Its molecular mass, as determined by a gel-filtration method is approximately 150,000 Da. According to SDS-PAGE WGeIF-2 consists of four subunits with M(r) 37,000 (alpha), 40,000 (beta), 42,000 (gamma) and 52,000 (delta). The beta- and gamma-subunits (but not the alpha-subunit) of WGeIF-2 can be readily phosphorylated by the double-stranded RNA activated kinase isolated from rabbit reticulocytes. Dissociation constants for WGeIF-2 complexes with GDP and GTP were measured. In our evaluation the WGeIF-2 affinity for GDP (KdGDP = 1.5 x 10(-7) M) was only 10 times higher than for GTP (KdGTP = 1.5 x 10(-6) M), while for rabbit reticulocyte eIF-2 (RReIF-2) the difference has been estimated as as much as two orders of magnitude in accordance with the literature. Close values of dissociation constants for WGeIF-2 complexes with guanine nucleotides suggest that at a sufficiently high [GTP]/[GDP] ratio the nucleotide exchange in wheat cells may take place without the participation of specific factor (eIF-2B) which catalyzes the nucleotide exchange on eIF-2 from mammalian cells.

The ATP requirement for initiation of eukaryotic translation varies according to the mRNA species

The requirement for ATP for initiation of eukaryotic mRNA translation was tested using gel-filtered rabbit reticulocyte lysates incubated with labelled Met-tRNAfMet and exogenous RNA templates, and assaying the formation of labelled 80S initiation complexes in the presence of GTP, or labelled 40S initiation complexes in the presence of a non-hydrolysable analogue of GTP. Initiation complex formation on globin mRNA, or on capped viral RNAs such as papaya mosaic virus RNA and tobacco mosaic virus RNA, was strongly stimulated by ATP. In contrast, initiation complex formation on (uncapped) encephalomyocarditis virus RNA was uninfluenced by the presence or absence of ATP, which may be correlated with the recent evidence for scanning-independent internal initiation on this viral RNA. In addition, initiation complex formation on uncapped cowpea mosaic virus RNA and on poly(A,U,G) was only slightly stimulated by ATP, much less than in the case of the capped RNAs. These results suggest that most of the ATP hydrolysed during translation initiation is consumed in cap-dependent processes, probably in unwinding the mRNA, and relatively little in the actual migration or scanning of 40S subunits along the mRNA.

Micropreparative separation of transfer ribonucleic acids by high-performance liquid chromatography

A method was developed for the micropreparative separation of individual species of tRNA using reversed-phase high-performance liquid chromatography on large pore spherical silica bonded with C3 alkyl chains. Columns were eluted with linear gradients of decreasing sodium chloride and increasing methanol concentrations. The decreasing salt gradient gradually abolished hydrophobic interactions and a significantly higher selectivity was thus obtained when compared with increasing gradients of salts usually employed in reversed-phase separations of tRNA. The acceptance of tRNA fractions was tested by charging them with fifteen different amino acids. Significantly different separations were obtained with tRNA from Escherichia coli and from rat liver. tRNAGlu and tRNATyr from E. coli were obtained in a pure form, all other tRNAs were more or less contaminated by adjoining tRNAs for other amino acids. Rechromatography under suitable isocratic conditions was required to obtain pure tRNA species from rat liver. Isoaccepting tRNAs for several amino acids were separated from rat liver. The method described seems suitable for preliminary fractionations of complex mixtures of tRNA and for a simple purification of isoaccepting species if the presence of tRNAs for other amino acids is not an hindrance.

Distinct binding sites for zinc and double-stranded RNA in the reovirus outer capsid protein sigma 3

By atomic absorption analysis, we determined that the reovirus outer capsid protein sigma 3, which binds double-stranded RNA (dsRNA), is a zinc metalloprotein. Using Northwestern blots and a novel zinc blotting technique, we localized the zinc- and dsRNA-binding activities of sigma 3 to distinct V8 protease-generated fragments. Zinc-binding activity was contained within an amino-terminal fragment that contained a transcription factor IIIA-like zinc-binding sequence, and dsRNA-binding activity was associated with a carboxy-terminal fragment. By these techniques, new zinc- and dsRNA-binding activities were also detected in reovirus core proteins. A sequence similarity was observed between the catalytic site of the picornavirus proteases and the transcription factor IIIA-like zinc-binding site within sigma 3. We suggest that the zinc- and dsRNA-binding activities of sigma 3 may be important for its proposed regulatory effects on viral and host cell transcription and translation.

Distinct binding sites for zinc and double-stranded RNA in the reovirus outer capsid protein sigma 3

By atomic absorption analysis, we determined that the reovirus outer capsid protein sigma 3, which binds double-stranded RNA (dsRNA), is a zinc metalloprotein. Using Northwestern blots and a novel zinc blotting technique, we localized the zinc- and dsRNA-binding activities of sigma 3 to distinct V8 protease-generated fragments. Zinc-binding activity was contained within an amino-terminal fragment that contained a transcription factor IIIA-like zinc-binding sequence, and dsRNA-binding activity was associated with a carboxy-terminal fragment. By these techniques, new zinc- and dsRNA-binding activities were also detected in reovirus core proteins. A sequence similarity was observed between the catalytic site of the picornavirus proteases and the transcription factor IIIA-like zinc-binding site within sigma 3. We suggest that the zinc- and dsRNA-binding activities of sigma 3 may be important for its proposed regulatory effects on viral and host cell transcription and translation.

The effect of serum deprivation on the initiation of protein synthesis in mouse neuroblastoma cells

Growth of mouse neuroblastoma cells becomes stationary when cultured in serum-free medium. Within 60 h, the protein-synthesizing capacity of the cells declines to 25% as compared to that of exponentially growing cells. The transitional activity of the crude ribosomal salt washes from serum-deprived and control cells was compared in in vitro protein-synthesizing pH 5 systems. It appears that the ribosomal salt wash from serum-deprived cells has significantly (70%) lost its ability to support the translation of neuroblastoma poly(A)+ RNA. This activity of the ribosomal wash from serum-deprived cells can be restored to control level with rabbit reticulocyte initiation factor eIF-4B only. The ability of the ribosomal wash from serum-deprived cells to support the translation of encephalomyocarditis virus (EMC) and Semliki Forest virus (SFV) 42 S mRNA was tested. We found that EMC-mRNA is efficiently translated with the ribosomal salt wash from serum-deprived cells, whereas on the other hand the translation of SFV 42 S mRNA is severely impaired. Therefore, we conclude that in serum-deprived neuroblastoma cells protein synthesis is regulated in both a quantitative and a qualitative way. Modulation of the activity of initiation factor of protein synthesis eIF-4B is at least partly responsible for the observed (selective) blockade of protein synthesis in serum-deprived cells.

Microsome-mediated stimulation of the aminoacylation of rat liver initiator transfer ribonucleic acid by benzo[alpha]pyrene and 3,3-dimethyl-1-phenyltriazene

The charging of initiator tRNAmetF with L-methionine was significantly stimulated by pre-incubation of unfractionated tRNA from rat liver with benzo [alpha]pyrne (BP) or 3,3-dimethyl-1-phenyltriazine (DMPT). The presence of microsomal enzymes from rat liver and of NADPH was absolutely required for this effect. Maximum enhancement was obtained after 60 min of incubation with 10(-4)-10(-8) mumol/ml of either compound tested. It appears that either proximate carcinogen, BP or DMPT, must be converted by microsomal enzymes into its direct-acting metabolites (ultimate carcinogens) which interact with initiator tRNA and thereby specifically modulate its aminoacylation.

Peptide-chain initiation in duodenal mucosa of rachitic chicks after 1 alpha-,25-dihydroxycholecalciferol administration

The post-ribosomal fraction of chick duodenal mucosa contains Met-tRNAMetf-binding protein(s) that behaves like the eukaryotic initiation factor (eIF-2) in protein synthesis. The binding activity of cytosol protein can be measured by retention of the radioactive complex formed on a nitrocellulose membrane. Complex-formation requires Met-tRNAMetf and GTP or guanosine [beta, gamma-methylene] triphosphate, and is inhibited by aurintricarboxylic acid. The ternary initiation complex thus formed can bind to ribosomal particles from chick intestine. By sucrose-density-gradient centrifugation, [35S]Met-tRNAMetf was found to bind exclusively to 40S and not to 60S ribosomal subunit particles. In the duodenal mucosa of rachitic chicks the ability of the cytosol proteins to promote the binding of Met-tRNAMetf to ribosomal particles via ternary-complex formation is detectably increased by 3 h after injection of 1 alpha,25-dihydroxycholecalciferol, the active form of vitamin D. Cholecalciferol and ergocalciferol under the same experimental conditions failed to stimulate Met-tRNAMetf-binding activity.

Assembly in vitro of a spanning membrane protein of the endoplasmic reticulum: the E1 glycoprotein of coronavirus mouse hepatitis virus A59

The E1 glycoprotein of coronavirus mouse hepatitis virus A59 was synthesized in vitro by translation of viral mRNA in the presence of dog pancreatic microsomes. Its disposition in the membrane was investigated by digestion with proteases and by selective NH2-terminal labeling. The protein spans the membrane, but only small portions from the NH2 and COOH terminus are exposed respectively in the lumenal and cytoplasmic domains; the bulk of the molecule is apparently buried in the membrane. The protein lacks a cleavable leader sequence and does not acquire its characteristic O-linked oligosaccharides in rough microsomes. It may enter the membrane at any stage during synthesis of the first 150 amino acid residues. These unusual features of the protein might help to explain why it is not transported to the cell surface in vivo but remains in intracellular membranes, causing the virus to bud there.

Avian sarcoma virus gag and env gene structural protein precursors contain a common amino-terminal sequence

The initiation site for translation of the avian sarcoma virus glycoprotein precursor, Pr63env, has been determined by analyzing the amino-terminal peptides of Pr63env and the polyprotein precursor Pr76gag encoded by the viral gag gene. The acceptor splice junction used to form the env gene mRNA has also been identified. Hybrid-selected virus-specific mRNAs were translated in vitro in the presence of either L-[35S]methionine to label at every methionine residue or L-[35S]methionine-tRNAMeti to label specifically at the amino-terminal methionine residues. Tryptic peptide maps of Pr63env labeled at every methionine residue contain all of the peptides, plus one additional peptide, present in the map of Pr57env, a nonglycosylated env-encoded polypeptide of molecular weight 57,000 immunoprecipitated from tunicamycin-treated cells. Specific amino-terminal labeling of the in vitro-synthesized polypeptides showed that the peptide missing from Pr57env corresponds to the amino-terminal tryptic peptide of Pr63env, which is removed in vivo as part of the amino-terminal signal peptide. Comparison of the amino-terminal tryptic peptides of Pr63env and Pr76gag showed that they are identical. In contrast, the chymotryptic amino-terminal peptides of Pr76gag and Pr63env are not identical. The location of the acceptor-splice junction in the env mRNA of the Prague A strain of avian sarcoma virus was determined by mung bean nuclease mapping to be at nucleotide 5,078. Fusion of the gag and env gene sequences during splicing results in use of the same AUG codon to initiate synthesis of Pr76gag and Pr63env. This sequence is contained within the 397-nucleotide 5' terminal leader that is spliced to the body of the env mRNA. The possible significance of these results for the regulation of avian sarcoma virus synthesis and translation is discussed.

Age-related changes in different steps of protein synthesis of liver and kidney of rats

Protein synthesis in cell-free systems of rat liver and kidney decreases markedly with age. Examination of activity changes of the different steps revealed for both types of organs that reduced binding of aminoacyl-tRNA to ribosomes and reduced peptidyl transfer might be of major importance for the decrease in overall protein synthesis whereas ageing has only little effect on translocation as well as on initiation and termination.

Biosynthesis of rabbit serum albumin in a heterologous fractionated subcellular system

As demonstrated by a simple procedure based on indirect immunoprecipitation, proteins retained on heparin-Sepharose 4B from postmitochondrial supernatants of rat liver and Zajdela hepatoma catalyse the translation of rabbit serum albumin mRNA in the presence of ribosomal subunits from rat liver, Zajdela hepatoma or rabbit reticulocytes. The albumin synthesis shows an optimum at 1.5 mM MgCl2 and 25 mM KCl and requires ATP and GTP. It is significantly stimulated by tRNA and proceeds for more than 2 h, suggesting a high rate of reinitiation. At the optimum ribosomes:mRNA ratio of 13:1, the immunoprecipitable radioactivity exceeds 15-20-times the blank values. Fluorography of polyacrylamide slabs after electrophoresis of immunoprecipitates revealed the presence of only complete full-size serum albumin without any smaller peptides resulting from premature terminations of polypeptide chains, demonstrating faithful translation. In stained gels only, both heavy and light chains of immunoglobulin G were found, indicating that the assay procedure is highly specific and reliable. The fractionated heterologous protein-synthesizing system described in this paper may be generally useful for studies on the synthesis of specific proteins and factors affecting their rates since, unlike comparable translation assays, a precise calculation of the balance of newly synthesized proteins is possible.

Processing of the encephalomyocarditis virus capsid precursor protein studied in rabbit reticulocyte lysates incubated with N-formyl-[35S]methionine-tRNAfMet

Translation of encephalomyocarditis virus RNA in rabbit reticulocyte lysates in the presence of N-formyl-[(35)S]methionine-tRNA(f) (Met) revealed that a small polypeptide is cleaved from the N-terminus of the capsid protein precursor, preA, by virus-coded protease activity. Therefore, this N-terminal segment comprising the translation initiation site is not conserved in any of the mature capsid proteins.

Inhibition of host translation in encephalomyocarditis virus-infected L cells: a novel mechanism

Encephalomyocarditis virus induced a rapid shutoff of host translation in mouse L cells shortly after infection and before viral proteins were made in detectable amounts. This kinetic pattern is similar to that seen in poliovirus-infected HeLa cells. However, the mechanisms of host shutoff are different in these two cases, for no reduction in the ability of lysates from encephalomyocarditis virus-infected L cells to translate capped mRNAs was observed. Instead, a change in the subcellular distribution of one or more initiation factors was seen. In particular, cap recognition activity in the high-speed supernatant fraction (S200) prepared from cell lysates increased threefold as a result of virus infection. The significance of this observation in terms of possible shutoff mechanisms is discussed. Inasmuch as the rapid host shutoff is not induced in at least four other cell types by encephalomyocarditis virus infection, it may be concluded that host shutoff mechanisms not only vary within the picornavirus group, but also depend upon the particular cell type employed.

Structural analysis of virion proteins of the avian coronavirus infectious bronchitis virus

We have found six major polypeptides in virions of the avian coronavirus infectious bronchitis virus grown in tissue culture: four glycoproteins, GP84, GP36, GP31, and GP28, and two non-glycosylated proteins, P51 and P23. In addition, we detected three minor species: two glycoproteins, GP90 and GP59, and one non-glycosylated protein, P14. Two-dimensional tryptic peptide mapping showed that GP36, GP31, GP28, and P23 comprise a group of closely related proteins which we have designated the "P23 family," but that the other proteins are distinct. Analysis by partial proteolytic digestion of P23 family, but that the other proteins are distinct. Analysis by partial proteolytic digestion of the P23 family labeled biosynthetically with [35S] methionine, and P23, labeled with [35S] formyl-methionine by in vitro translation of RNA from infected cells, revealed that the proteins of the P23 family differ in their amino-terminal domains. Similar analysis of GP31 and Gp36 labeled with [3H] mannose showed that the partial proteolytic fragments unique to these proteins were glycosylated. This suggests that differences in glycosylation in the amino-terminal domains contributes to the marked polymorphism os the P23 family. The results are discussed with respect to possible models for synthesis of the virion proteins.

The protein synthetic activity in vitro of ribosomes differing in the extent of phosphorylation of their ribosomal proteins

We describe a re-examination of the cell-free protein synthetic activity of eukaryotic ribosomes having proteins phosphorylated to different extents. Ribosomal 40 S subunits were isolated both from a variety of cells in which there is relatively little phosphorylation of ribosomal protein S6, and from cells subjected in vivo to different stimuli that promote the extensive phosphorylation of protein S6. The ability of these subunits to bind Met-tRNA as well as the second amino acyl-tRNA (Val-tRNA) was compared in the presence of highly purified initiation factors, elongation factor EF-1 at various concentrations of 60S subunits, 9 S globin mRNA and potassium ions. The ability of the subunits to synthesize polyphenylalanine was also studied using highly purified elongation factors. In no case was any significant difference in activity observed between ribosomes with protein S6 phosphorylated to different extents. Similar, though less extensive, studies were preformed comparing 60 S ribosomal subunits differing in the extent of phosphorylation of the acidic phosphoprotein, L gamma , and of L14. No difference in activity was observed between these ribosomes.

Particulate aminoacyl-tRNA synthetases are retained on heparin bound to Sepharose

Fractions of reticulocyte lysates were retained on heparin immobilized on Sepharose 4B and further separated by gel filtration on Sepharose 6B. These fractions contain aminoacyl-tRNA synthetases for 12 amino acids tested. Most synthetases with the highest specific activity are present in the fraction of approximately 40S. Particulate synthetases present in the postmicrosomal pellet of rat liver and synthetases associated with polyribosomes are almost completely adsorbed on heparin-Sepharose but free enzymes in the cytosol are not retained. Since only particulate synthetases are retained on the affinity carrier, their adsorption may be due to the presence of protein-synthesis factors, in particular peptide initiation and elongation factors, in the complexes of synthetases.

Incorporation of labelled amino acids into proteins, from rabbit reticulocytes, retained on heparin-sepharose

Fractions of rabbit reticulocyte lysates retained on heparin-Sepharose 4B catalyze incorporation of labelled amino acids into proteins in the absence of ribosomes, and several characteristics of this reaction are identical with peptide bond formation mediated by aminoacyl-tRNA-protein transferases. At least five different proteins become labelled, as revealed by polyacrylamide gel electrophoresis. Active fractions synthesize aminoacyl-tRNA which is utilized by transferase. Aminoacyl-tRNA-protein transferase activity may be uncoupled from that of aminoacyl-tRNA synthetase by puromycin or lysyl-phenylalanine which both inhibit the transferase activity only. Adenosine and phosphate inhibit aminoacyl-tRNA synthetase as well as the incorporation of labelled amino acids into proteins. This indicates that the incorporation must be preceded by charging of tRNA with amino acids. Presence of three different aminoacyl-tRNA-protein transferases, each of them specific for a group of four amino acids, was demonstrated. Serum albumin stimulates the incorporation of amino acids and a labelling of this protein was demonstrated in mixtures into which it had been added. Addition of both ribosomal subunits and globin messenger ribonucleoprotein significantly changes the pattern of labelled proteins, and synthesis of globin was demonstrated by polyacrylamide gel electrophoresis. Density-gradient analysis revealed formation of a 48 S ribosomal complex and the formation of polyribosomes. Systems composed of active fractions retained on heparin-Sepharose supplemented with ribosomes and globin messengers apparently catalyze the translation of this message, but interactions do exist between the ribosome-mediated peptide synthesis and non-ribosomal incorporation of amino acids into proteins.

Cytoplasmically made subunits of yeast mitochondrial F1-ATPase and cytochrome c oxidase are synthesized as individual precursors, not as polyproteins

At least three subunits of yeast mitochondrial F1-ATPase (ATP phosphohydrolase, EC 3.6.1.3) and at least two subunits of cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) are synthesized outside the mitochondria and imported into the organelles as individual precursors that are between 2000 and 6000 daltons larger than the mature subunits. These precursors were shown to be primary translation products. Therefore, neither the five F1 subunits nor the four small cytochrome c oxidase subunits are synthesized as a single polyprotein.

The mechanism of action of eukaryotic initiation factor 4C in protein synthesis

Binding of eukaryotic initiation factor 4C (eIF-4C) to a 40-S initiation complex could be detected by analysis of assay mixtures for 40-S initiation complex formation on columns of Sepharose 6B and on sucrose gradients. On Sepharose 6B columns an almost stoichiometric binding of [14C]eIF-4C was observed, provided that eIF-1, eIF-2, eIF-3, Met-tRNAf and mRNA were included in the assay mixtures. On sucrose gradients the presence of 0.5 microM edeine was an additional prerequisite for stable binding of eIF-4C. The presence of eIF-4C on the 40-S initiation complex resulted in a pronounced increase of its stability, which might explain the observed stimulatory effect of the factor on the binding of Met-tRNAf into initiation complexes. The interaction between eIF-4C and the small ribosomal subunit presumably occurs at an early stage of the initiation process, since eIF-4C was capable of dissociating 80-S ribosomes into subunits. Protein synthesis in a 'pH-5' assay was almost totally dependent on the addition of eIF-4C, when the assay was performed with purified, nearly homogeneous, initiation factors.

Purification of free eukaryotic initiation factors eIF-4A and eIF-4D on Cibacron Blue F3G-A

The eukaryotic initiation factors eIF-4A and eIF-4D are almost exclusively found in the supernatant of a reticulocyte lysate. The basic steps of general purification schemes failed because these factors do not bind to Sepharoseheparin or, in the of eIF-4A, to phosphocellulose and new procedure had to be devised. Cibacron Blue F3G-A proved to be a successful alternative in the purification of these factors from the excessive load of proteins found in the supernatant.

Cell-free synthesis of the mitochondrial ADP/ATP carrier protein of Neurospora crassa

ADP/ATP carrier protein was synthesized in heterologous cell-free systems programmed with Neurospora poly(A)-containing RNA and homologous cell-free systems from Neurospora. The apparent molecular weight of the product obtained in vitro was the same as that of the authentic mitochondrial protein. The primary translation product obtained in reticulocyte lysates starts with formylmethionine when formylated initiator methionyl-tRNA (fMet-tRNAfMet) was present. The product synthesized in vitro was released from the ribosomes into the postribosomal supernatant. The evidence presented indicates that the ADP/ATP carrier is synthesized as a polypeptide with the same molecular weight as the mature monomeric protein and does not carry an additional sequence.

A comparison of the initiation factors of eukaryotic protein synthesis from ribosomes and from the postribosomal supernatant

A purification procedure is described for the initiation factors of protein synthesis from rabbit reticulocytes: (a) from the ribosomal wash and (b) from the postribosomal supernantant. A comparison is made between these preparations with respect to yield and specific activity. eIF-4A and eIF-4D occur mainly in the postribosomal supernatant; eIF-2, eIF-4C and eIF-5 are more evenly divided over both fractions, whereas eIF-1, eIF-3 and eIF-4B are found almost exclusively in the ribosomal wash. No significant difference in specific activity could be detected when factors from both sources were compared, with a possible exception of eIF-4A and eIF-4D.

Translation of encephalomyocarditis virus RNA in vitro yields an active proteolytic processing enzyme

In contrast to other cell-free translation systems, the mRNA-dependent reticulocyte lysate can translate encephalomyocarditis virus RNA efficiently and completely when supplemented with heterologous tRNA. Cleavage of the nascent polypeptide chain occurs, and one of the translation products appears to be a specific proteolytic enzyme which correctly processes the primary products. The identity of the proteins made in vitro was verified by comparison with infected cell proteins on dodecylsulphate/polyacrylamide gels, and by mapping their coding sequences on the viral genome.

All factors required for protein synthesis are retained on heparin bound to Sepharose

1. Postmitochondrial supernatants of rabbit reticulocyte lysates were chromatographed on heparin bound to Sepharose 4B, and the fraction retained on affinity columns was separated by subsequent gel filtration on Sepharose 4B into three fractions, two of them active in protein synthesis. 2. The heavier fraction sedimented at 40S and contained more than 10% RNA. This consisted predominantly of a 12S component, with smaller amounts of the 9S and 4S RNA species. The lighter fraction (18-20S) was composed of proteins with less than 1% RNA. 3. Different enzymic activities were associated with these fractions. 4. In the presence of both fractions, efficient translation took place on combined ribosomal subunits of rat liver with added cofactors. Globin messenger ribonucleoprotein stimulated this translation 5-6-fold. 5. Relatively large complexes of all factors required for protein synthesis are apparently isolated from reticulocytes by affinity chromatography on heparin-Sepharose 4B. Such complexes may occur naturally in the cytoplasm of mammalian cells.