Cytoplasmic methionine transfer RNAs from eukaryotes

Highly chromophoric Cy5-methionine for N-terminal fluorescent tagging of proteins in eukaryotic translation systems

Despite significant advances on fluorescent labeling of target proteins to study their structural dynamics and function, there has been need for labeling with high quantum yield ensuring high sensitivity and selectivity without sacrificing the biological function of the protein. Here as a technical advancement over non-canonical amino acid incorporation, we provided a conceptual design of the N-terminal fluorescent tagging of proteins. Cy5-labeled methionine (Cy5-Met) was chemically synthesized, and then the purified Cy5-Met was coupled with synthetic human initiator tRNA by methionine tRNA synthetase. Cy5-Met-initiator tRNA (Cy5-Met-tRNAi) was purified and transfected into HeLa cells with HIV-Tat plasmid, resulting in an efficient production of Cy5-labeled HIV-Tat protein. Based on the universal requirement in translational initiation, the approach provides co-translational incorporation of N-terminal probe to a repertoire of proteins in the eukaryote system. This methodology has potential utility in the single molecule analysis of human proteins in vitro and in vivo for addressing to their complex biological structural and functional dynamics.

Mutant methionyl-tRNA synthetase from bacteria enables site-selective N-terminal labeling of proteins expressed in mammalian cells

Newly synthesized cellular proteins can be tagged with a variety of metabolic labels that distinguish them from preexisting proteins and allow them to be identified and tracked. Many such labels are incorporated into proteins via the endogenous cellular machinery and can be used in numerous cell types and organisms. Though broad applicability has advantages, we aimed to develop a strategy to restrict protein labeling to specified mammalian cells that express a transgene. Here we report that heterologous expression of a mutant methionyl-tRNA synthetase from Escherichia coli permits incorporation of azidonorleucine (Anl) into proteins made in mammalian (HEK293) cells. Anl is incorporated site-selectively at N-terminal positions (in competition with initiator methionines) and is not found at internal sites. Site selectivity is enabled by the fact that the bacterial synthetase aminoacylates mammalian initiator tRNA, but not elongator tRNA. N-terminally labeled proteins can be selectively conjugated to a variety of useful probes; here we demonstrate use of this system in enrichment and visualization of proteins made during various stages of the cell cycle. N-terminal incorporation of Anl may also be used to engineer modified proteins for therapeutic and other applications.

Protein Chain-Initiating Methionine tRNAs in Chloroplasts and Cytoplasm of Wheat Leaves

The role of methionine in protein chain initiation in wheat has been studied. Two chain-initiating methionine tRNAs have been found. One of these is located in the cytoplasm of the wheat cell. This methionyl-tRNA is not formylated by wheat extracts and appears to function in protein chain initiation in the cytoplasm without prior formylation. The other initiating tRNA is from chloroplasts. This methionyl-tRNA is formylated by a transformylase present in wheat extracts and functions in chain initiation in chloroplasts as formylmethionyl-tRNA.

The many applications of acid urea polyacrylamide gel electrophoresis to studies of tRNAs and aminoacyl-tRNA synthetases

Here we describe the many applications of acid urea polyacrylamide gel electrophoresis (acid urea PAGE) followed by Northern blot analysis to studies of tRNAs and aminoacyl-tRNA synthetases. Acid urea PAGE allows the electrophoretic separation of different forms of a tRNA, discriminated by changes in bulk, charge, and/or conformation that are brought about by aminoacylation, formylation, or modification of a tRNA. Among the examples described are (i) analysis of the effect of mutations in the Escherichia coli initiator tRNA on its aminoacylation and formylation; (ii) evidence of orthogonality of suppressor tRNAs in mammalian cells and yeast; (iii) analysis of aminoacylation specificity of an archaeal prolyl-tRNA synthetase that can aminoacylate archaeal tRNA(Pro) with cysteine, but does not aminoacylate archaeal tRNA(Cys) with cysteine; (iv) identification and characterization of the AUA-decoding minor tRNA(Ile) in archaea; and (v) evidence that the archaeal minor tRNA(Ile) contains a modified base in the wobble position different from lysidine found in the corresponding eubacterial tRNA.

The naturally occurring N6-threonyl adenine in anticodon loop of Schizosaccharomyces pombe tRNAi causes formation of a unique U-turn motif

Modified nucleosides play an important role in structure and function of tRNA. We have determined the solution structure of the anticodon stem-loop (ASL) of initiator tRNA of Schizosaccharomyces pombe. The incorporation of N6-threonylcarbamoyladenosine at the position 3' to the anticodon triplet (t6A37) results in the formation of a U-turn motif and enhances stacking interactions within the loop and stem regions (i.e. between A35 and t6A37) by bulging out U36. This conformation was not observed in a crystal structure of tRNAi including the same modification in its anticodon loop, nor in the solution structure of the unmodified ASL. A t6A modification also occurs in the well studied anti-stem-loop of lys-tRNA(UUU). A comparison of this stem-loop with our structure demonstrates different effects of the modification depending on the loop sequence.

Importance of the anticodon sequence in the aminoacylation of tRNAs by methionyl-tRNA synthetase and by valyl-tRNA synthetase in an Archaebacterium

The mode of recognition of tRNAs by aminoacyl-tRNA synthetases and translation factors is largely unknown in archaebacteria. To study this process, we have cloned the wild type initiator tRNA gene from the moderate halophilic archaebacterium Haloferax volcanii and mutants derived from it into a plasmid capable of expressing the tRNA in these cells. Analysis of tRNAs in vivo show that the initiator tRNA is aminoacylated but is not formylated in H. volcanii. This result provides direct support for the notion that protein synthesis in archaebacteria is initiated with methionine and not with formylmethionine. We have analyzed the effect of two different mutations (CAU-->CUA and CAU-->GAC) in the anticodon sequence of the initiator tRNA on its recognition by the aminoacyl-tRNA synthetases in vivo. The CAU-->CUA mutant was not aminoacylated to any significant extent in vivo, suggesting the importance of the anticodon in aminoacylation of tRNA by methionyl-tRNA synthetase. This mutant initiator tRNA can, however, be aminoacylated in vitro by the Escherichia coli glutaminyl-tRNA synthetase, suggesting that the lack of aminoacylation is due to the absence in H. volcanii of a synthetase, which recognizes the mutant tRNA. Archaebacteria lack glutaminyl-tRNA synthetase and utilize a two-step pathway involving glutamyl-tRNA synthetase and glutamine amidotransferase to generate glutaminyl-tRNA. The lack of aminoacylation of the mutant tRNA indicates that this mutant tRNA is not a substrate for the H. volcanii glutamyl-tRNA synthetase. The CAU-->GAC anticodon mutant is most likely aminoacylated with valine in vivo. Thus, the anticodon plays an important role in the recognition of tRNA by at least two of the halobacterial aminoacyl-tRNA synthetases.

Formylation is not essential for initiation of protein synthesis in all eubacteria

Formylation of the initiator methionyl-tRNA, catalyzed by methionyl-tRNA formyltransferase, has long been regarded as essential for initiation of protein synthesis in eubacteria. Here, we show that this process is, in fact, dispensable in Pseudomonas aeruginosa. Disruption of the chromosomal methionyl-tRNA formyltransferase gene in P. aeruginosa resulted only in a moderate decrease in the rate of cell growth, whereas in Escherichia coli cell growth was severely impaired. The ability of the P. aeruginosa mutant strain to grow was not due to an additional copy of the methionyl-tRNA formyltransferase gene or to N-acylation of the methionyl moiety by a group other than formyl. These results indicate that P. aeruginosa can carry out formylation-independent initiation of protein synthesis, using the nonformylated methionyl-tRNA. Therefore, the dogma that eubacteria require formylation of the initiator methionyl-tRNA for initiation of protein synthesis may have been an invalid generalization of results obtained with E. coli.

Initiator-elongator discrimination in vertebrate tRNAs for protein synthesis

Initiator tRNAs are used exclusively for initiation of protein synthesis and not for the elongation step. We show, in vivo and in vitro, that the primary sequence feature that prevents the human initiator tRNA from acting in the elongation step is the nature of base pairs 50:64 and 51:63 in the TpsiC stem of the initiator tRNA. Various considerations suggest that this is due to sequence-dependent perturbation of the sugar phosphate backbone in the TpsiC stem of initiator tRNA, which most likely blocks binding of the elongation factor to the tRNA. Because the sequences of all vertebrate initiator tRNAs are identical, our findings with the human initiator tRNA are likely to be valid for all vertebrate systems. We have developed reporter systems that can be used to monitor, in mammalian cells, the activity in elongation of mutant human initiator tRNAs carrying anticodon sequence mutations from CAU to CCU (the C35 mutant) or to CUA (the U35A36 mutant). Combination of the anticodon sequence mutation with mutations in base pairs 50:64 and 51:63 yielded tRNAs that act as elongators in mammalian cells. Further mutation of the A1:U72 base pair, which is conserved in virtually all eukaryotic initiator tRNAs, to G1:C72 in the C35 mutant background yielded tRNAs that were even more active in elongation. In addition, in a rabbit reticulocyte in vitro protein-synthesizing system, a tRNA carrying the TpsiC stem and the A1:U72-to-G1:C72 mutations was almost as active in elongation as the elongator methionine tRNA. The combination of mutant initiator tRNA with the CCU anticodon and the reporter system developed here provides the first example of missense suppression in mammalian cells.

A novel pathway for the conversion of homocysteine to methionine in eukaryotes

Activation of amino acid homocysteine was compared with that of methionine in rabbit crude liver extracts and purified multi-enzyme complex of aminoacyl-tRNA synthetases. Activation was studied by measuring the incorporation of radioactive amino acid into unlabelled trichloroacetic-acid insoluble materials in the absence of protein synthesis. Homocysteine synthetase activity was found in the crude extract and in the purified multi-enzyme complex of aminoacyl-tRNA synthetases. On a molar basis, the activation of methionine by the crude extract was five times higher than the activation of homocysteine. There was a partial loss of Hcy-tRNA synthetase activity in the purified multi-enzyme complex. Preliminary reconstitution experiments indicated a requirement for an additional factor for Hcy-tRNA synthetase activity. TLC of the amino acid released from tRNA charged with [14C]homocysteine, revealed radioactivity in homocysteine, methionine and homocysteine thiolactone, indicating a conversion of tRNA-attached homocysteine to methionine. Total tRNA was separated on a benzoylated cellulose column into a fraction enriched in initiator tRNA and a methionine-accepting, but initiator tRNA-deficient, fraction. Homocysteine-accepting activity was present only in the initiator tRNA-enriched fraction. Based on the above data we propose that homocysteine activation in reticulocyte lysates, reported previously, also occurs in liver. Activated homocysteine is attached to initiator tRNA and then converted to methionine by a methylating enzyme. In the absence of methylation, tRNA-attached homocysteine is hydrolysed to produce homocysteine thiolactone.

The A1 x U72 base pair conserved in eukaryotic initiator tRNAs is important specifically for binding to the eukaryotic translation initiation factor eIF2

The formation of a specific ternary complex between eukaryotic initiation factor 2 (eIF2), the initiator methionyl-tRNA (Met-tRNA), and GTP is a critical step in translation initiation in the cytoplasmic protein-synthesizing system of eukaryotes. We show that the A1 x U72 base pair conserved at the end of the acceptor stem in eukaryotic and archaebacterial initiator methionine tRNAs plays an important role in this interaction. We changed the A1 x U72 base pair of the human initiator tRNA to G1 x C72 and expressed the wild-type and mutant tRNA genes in the yeast Saccharomyces cerevisiae by using constructs previously developed in our laboratory for expression of the human initiator tRNA gene in yeasts. We show that both the wild-type and mutant human initiator tRNAs are aminoacylated well in vivo. We have isolated the wild-type and mutant human initiator tRNAs in substantially pure form, free of the yeast initiator tRNA, and have analyzed their properties in vitro. The G1 x C72 mutation affects specifically the binding affinity of eIF2 for the initiator tRNA. It has no effect on the subsequent formation of 40S or 80S ribosome initiator Met-tRNA-AUG initiation complexes in vitro or on the puromycin reactivity of the Met-tRNA in the 80S initiation complex.

Translation of encephalomyocarditis virus RNA by internal ribosomal entry

Picornavirus 5' NCRs contain IRES elements that have been divided into two groups, exemplified by PV (type 1) and EMCV (type 2). These elements are functionally related and have an intriguing level of structural and sequence similarity. Some conserved RNA sequences and/or structures may correspond to cis-acting elements involved in IRES function, so that there may also be similarities in the mechanism by which the two types or IRES promote initiation. The function of both types of IRES element appears to depend on a cellular 57 kDa polypeptide, which has been identified as the predominantly nuclear hnRNP protein PTB. However, a specific function for p57/PTB in translation has not yet been established. These two groups can be differentiated on the basis of their requirements for trans-acting factors. The EMCV IRES functions efficiently in a broader range of eukaryotic cell types than type 1 IRES elements, probably because the latter require additional factor(s). A second distinction between these IRES element is that initiation occurs directly at the 3' border of type 2 IRES elements, whereas a nonessential spacer of between 30 nt and 154 nt separates type 1 IRES elements from the downstream initiation codon.

Rit1, a tRNA backbone-modifying enzyme that mediates initiator and elongator tRNA discrimination

Using a genetic screen in yeast aimed at identifying cellular factors involved in initiator and elongator methionine tRNA discrimination in the translational process, we have identified a mutation that abolish the requirement for elongator methionine tRNA. The gene affected, which we call the ribosylation of the initiator tRNA gene or RIT1, encodes a 2'-O-ribosyl phosphate transferase. This enzyme modifies exclusively the initiator tRNA in position 64 using 5'-phosphoribosyl-1'-pyrophosphate as the modification donor. As the initiator tRNA participates both in the initiation and elongation of translation in a rit1 strain, we conclude that the 2'-O-ribosyl phosphate modification discriminates the initiator tRNAs from the elongator tRNAs during protein synthesis. The modification enzyme was shown to recognize the stem-loop IV region that is unique in eukaryotic cytoplasmic initiator tRNAs.

Mutational analysis of conserved positions potentially important for initiator tRNA function in Saccharomyces cerevisiae

The conserved positions of the eukaryotic cytoplasmic initiator tRNA have been suggested to be important for the initiation of protein synthesis. However, the role of these positions is not known. We describe in this report a functional analysis of the yeast initiator methionine tRNA (tRNA(iMet)), using a novel in vivo assay system which is not dependent on suppressor tRNAs. Strains of Saccharomyces cerevisiae with null alleles of the four initiator methionine tRNA (IMT) genes were constructed. Consequently, growth of these strains was dependent on tRNA(iMet) encoded from a plasmid-derived gene. We used these strains to investigate the significance of the conserved nucleosides of yeast tRNA(iMet) in vivo. Nucleotide substitutions corresponding to the nucleosides of the yeast elongator methionine tRNA (tRNA(MMet)) have been made at all conserved positions to identify the positions that are important for tRNA(iMet) to function in the initiation process. Surprisingly, nucleoside changes in base pairs 3-70, 12-23, 31-39, and 29-41, as well as expanding loop I by inserting an A at position 17 (A17) had no effect on the tester strain. Nucleotide substitutions in positions 54 and 60 to cytidines and guanosines (C54, G54, C60, and G60) did not prevent cell growth. In contrast, the double mutation U/rT54C60 blocked cell growth, and changing the A-U base pair 1-72 to a G-C base pair was deleterious to the cell, although these tRNAs were synthesized and accepted methionine in vitro. From our data, we suggest that an A-U base pair in position 1-72 is important for tRNA(iMet) function, that the hypothetical requirement for adenosines at positions 54 and 60 is invalid, and that a U/rT at position 54 is an antideterminant distinguishing an elongator from an initiator tRNA in the initiation of translation.

Mutational analysis of conserved positions potentially important for initiator tRNA function in Saccharomyces cerevisiae

The conserved positions of the eukaryotic cytoplasmic initiator tRNA have been suggested to be important for the initiation of protein synthesis. However, the role of these positions is not known. We describe in this report a functional analysis of the yeast initiator methionine tRNA (tRNA(iMet)), using a novel in vivo assay system which is not dependent on suppressor tRNAs. Strains of Saccharomyces cerevisiae with null alleles of the four initiator methionine tRNA (IMT) genes were constructed. Consequently, growth of these strains was dependent on tRNA(iMet) encoded from a plasmid-derived gene. We used these strains to investigate the significance of the conserved nucleosides of yeast tRNA(iMet) in vivo. Nucleotide substitutions corresponding to the nucleosides of the yeast elongator methionine tRNA (tRNA(MMet)) have been made at all conserved positions to identify the positions that are important for tRNA(iMet) to function in the initiation process. Surprisingly, nucleoside changes in base pairs 3-70, 12-23, 31-39, and 29-41, as well as expanding loop I by inserting an A at position 17 (A17) had no effect on the tester strain. Nucleotide substitutions in positions 54 and 60 to cytidines and guanosines (C54, G54, C60, and G60) did not prevent cell growth. In contrast, the double mutation U/rT54C60 blocked cell growth, and changing the A-U base pair 1-72 to a G-C base pair was deleterious to the cell, although these tRNAs were synthesized and accepted methionine in vitro. From our data, we suggest that an A-U base pair in position 1-72 is important for tRNA(iMet) function, that the hypothetical requirement for adenosines at positions 54 and 60 is invalid, and that a U/rT at position 54 is an antideterminant distinguishing an elongator from an initiator tRNA in the initiation of translation.

Regulation of protein synthesis in virus-infected animal cells

This chapter summarizes the structural features that govern the translation of viral mRNAs: where the synthesis of a protein starts and ends, how many proteins can be produced from one mRNA, and how efficiently. It focuses on the interplay between viral and cellular mRNAs and the translational machinery. That interplay, together with the intrinsic structure of viral mRNAs, determines the patterns of translation in infected cells. It also points out some possibilities for translational regulation that can only be glimpsed at present, but are likely to come into focus in the future. The mechanism of selecting the initiation site for protein synthesis appears to follow a single formula. The translational machinery displays a certain flexibility that is exploited more frequently by viral than by cellular mRNAs. Although some of the parameters that determine efficiency have been identified, how efficiently a given mRNA will be translated cannot be predicted by summing the known parameters.

Hemoglobin Long Island is caused by a single mutation (adenine to cytosine) resulting in a failure to cleave amino-terminal methionine

Hemoglobin Long Island has two separate amino acid abnormalities of beta-globin structure: an extension of the NH2 terminus by a methionine residue and a histidine-to-proline substitution at the normal second position. The NH2-terminal methionine residue, the translation product of an AUG initiation codon, is present only transiently in nascent proteins. Because of the general biological implications of this abnormality, we investigated the nature of the genetic defect of this mutant. We determined the sequence of the relevant portion of the beta-globin mRNA by means of dideoxynucleotide chain termination of the complementary DNA (cDNA) in which an oligonucleotide complementary to codons 10-17 was used as a primer for reverse transcriptase. A histidine-to-proline substitution was confirmed in the mutant mRNA by identifying an adenine-to-cytosine transversion in the second codon. However, we were unable to find any other abnormality at either the AUG initiation codon or in the 56 bases upstream from the adenine-to-cytosine transversion (encompassing most of the 5' untranslated region of the mutant beta-globin mRNA). Thus, it appears that this single lesion probably interferes with the poorly understood methionine-cleaving mechanism that modulates most of prokaryotic and eukaryotic proteins.

Linear mapping of tryptophan residues in Vesiculovirus M and N proteins by partial chemical cleavage

Nonlimit chemical cleavage at tryptophan residues of protein labeled at the amino terminus afforded a simple procedure for generating specific fragments and for mapping tryptophan positions. A comparison of the matrix (M) and nucleocapsid (N) proteins of four members of the Vesiculovirus group by this procedure suggests considerable conservation of tryptophan number and location in the four serotypes examined.

Messenger ribonucleoprotein complexes containing in vitro-synthesized 26S and 42S Semliki Forest virus RNA

An extract derived from Semliki Forest virus (SFV) infected cells is described which catalyzes the synthesis of virus-specific RNAs. The newly-synthesized 26S and 42S RNAs are found complexed with protein as messenger ribonucleoproteins (mRNPs). These mRNPs either are non-membrane bound or are associated with large cytoplasmic lipoprotein membranes, and they are found as free mRNPs as well as mRNPs bound to ribosomal subunits, ribosomes, and polysomes. Following treatment with Tween 40 and deoxycholate, membrane-bound mRNPs containing in vitro-synthesized 26S RNA are dissociated and sediment at 33S. These membrane-dissociated mRNPs contain relatively little protein. In contrast, the free or ribosome-bound mRNPs, which are isolated as 30S to 160S particles, remain heterogeneous after detergent treatment and have a much higher protein content. Addition of purified, native 40S ribosomal subunits to the extract leads to the formation of complexes between the added ribosomal subunits and the newly-synthesized viral mRNA. The in vitro-synthesized 26S and 42S RNAs participate in the assembly of translational initiation and elongation complexes.

Multiple intracellular peptidases in Neurospora crassa

Neurospora crassa possesses multiple intracellular peptidases which display overlapping substrate specificities. They were readily detected by an in situ staining procedure for peptidases separated in polyacrylamide gels, within which the auxilliary enzyme, l-amino acid oxidase, was immobilized. Eleven different intracellular peptidases were identified by electrophoretic separation and verified by their individual patterns of substrate specificities. Most peptide substrates tested were hydrolyzed by several different peptidases. The multiple intracellular peptidases may play overlapping roles in several basic cell processes which involve peptidase activity. The amount of peptidase activity for leucylglycine present in crude extracts of cells grown under widely different conditions was relatively constant, suggesting that this enzyme may be constitutive, although alterations in the amounts of individual peptidase isozymes may occur. A single enzyme, designated peptidase II, was partially purified and obtained free from the other peptidase species. Peptidase II was found to be an aminopeptidase with activity toward many peptides of varied composition and size. It was more active with tripeptides than homologous dipeptides and showed strong activity toward methionine-containing peptides. This enzyme, with a molecular weight of about 37,000, was thermolabile at 65 degrees C and was strongly inhibited by p-hydroxymercuribenzoate, Zn(2+), Co(2+), and Mn(2+), but was insensitive to the serine protease inhibitor phenylmethylsulfonyl fluoride. Peptidase II apparently possesses an essential sulfhydryl group and may be a metalloenzyme.

Evolutionary change in invertebrate cytochrome C

Recently published amino acid sequences are compared to those of other cytochromes c. Molecular phylogenies constructed by using an ancestral sequence method are compared to the classical biological view of invertebrate evolution. Problems associated with the analysis of sequences of different chain lengths and of high variability are discussed, and the logistics of increasing the representation of key invertebrate phyla is assessed.

Initiation of protein synthesis in bacillus subtilis in the presence of trimethoprim or aminopterin

Initiation of protein synthesis has been studied in the presence of the tetrahydrofolic acid analogues trimethoprim or aminopterin in Bacillus subtilis. This bacterium can grow in the presence of the inhibitors, when the medium is supplemented with the low molecular weight products of tetrahydrofolate-dependent pathways. In an attempt to show whether formylation of initiator tRNA is a prerequisite for the iniation of protein synthesis in procaryotic cells, the amount of N-formylmethionine in tRNA and in protein has been determined. The level of formylation of methionyl-tRNA was found to be 70% in control cells and approximately 2% in inhibitor-treated cells. The content of formyl groups in protein has also been found to be drastically reduced. Trimethoprim or aminopterin did not alter the amount of tRNAMet nor the degree of aminoacylation of tRNAMet in vivo. These results indicate that in B. subtilis inititation of protein synthesis is possible without prior formylation of initiator tRNA.

Studies on the formation and stability of aminoacyl-tRNA synthetase complexes from Ehrlich ascites cells

Nine aminoacyl-tRNA synthetases from Ehrlich ascites cells were examined with respect to their ability to be isolated as high molecular weight complexes, soluble enzymes, and ribosome-bound enzymes. Several different methods were employed for cell homogenization and enzyme isolation, with particular attention paid to the effects of hypotonic, isotonic, and hypertonic buffers on enzyme isolation. The binding of all synthetases to ribosomes was eliminated if the low ionic strength of the isolation buffer was raised to isotonic levels. In contrast, neither the ionic strength or composition of the buffers, nor the procedures used for cell homogenization or enzyme isolation had any significant effect on the isolation of the high molecular weight synthetase complex. Certain enzymes (lysyl-, methionyl- and isoleucyl-tRNA synthetases) formed very stable complexes and high molecular weight species were the predominant forms of these enzymes under all conditions of cell homogenization and enzyme isolation. Other enzymes (glycyl-, tyrosinyl- and threonyl-tRNA synthetases) formed complexes very weakly, if at all, and always appeared predominately in the soluble enzyme fraction. Isolated soluble forms of the lysyl-, methionyl- and isoleucyl-tRNA synthetases did not associate to form significant amounts of complex upon re-isolATION, SUGGESTING THAT A COMPONENT NECESSARY FOR COMPLEX FORMATION WAS MISSING FROM THE SOLUBLE ENZYME FRACTION. However, the soluble forms of these enzymes, but not the glycyl-, tyrosinyl- and threonyl-tRNA synthetases, did for complexes when mixed with ribosomal RNA or polyuridylic acid. Preliminary experiments showed no significant differences between the complexed and soluble forms of the lysyl-, methionyl- and isoleucyl-tRNA synthetases with respect to Km values or ability to charge different isoaccepting tRNAs.

The mechanism of inhibition of initiation of protein synthesis in reticulocyte lysates by pyrocatechol violet

1. Pyrocatechol violet inhibits the initiation of protein synthesis in reticulocyte lysates. It prevents the formation of 80-S initiation complexes by inhibiting the binding of mRNA to 40-S initiation complexes. 2. Pyrocatechol violet inhibits the hydrolysis of methionyl-tRNAmetf by reticulocyte lysates or reticulocyte cell sap. 3. Concentrations of pyrocatechol violet sufficient to inhibit initiation have little effect on amino acid activation. Higher concentrations of pyrocatechol violet were strongly inhibitory.

Macromolecule synthesis in a mutant of Saccharomyces cerevisiae inhibited by S-adenosyimethionine

Saccharomyces cerevisiae strain 83384-B3 carries the sai-1 mutation which confers sensitivity to S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH). It was shown that the mutant is impermeable to precursors of ribonucleic acid (RNA) and protein during inhibition by SAM (0.2 mM). Inhibition of uptake of adenine and uracil was nearly complete 3 h after growth in the presence of SAM and the uptake of leucine was at least 10-fold lower. The incorporation of 3H-adenine into ribosomal RNA, transfer RNA and heterodisperse RNA, believed to be messenger, was reduced 10-fold when measured after 1 h inhibition. The inhibition of growth was completely reversed by methionine (2.0 mM) in cells previously exposed to SAM for 90 min. The polysome content in cells inhibited by SAM was 25% less than the control after 4 h inhibition. Ribosome synthesis increased only about 40% in the presence of SAM and about 5-fold in the control over an 8 h period. All classes of RNA were synthesized during inhibition.

Purification and properties of two initiation factors from Bacillus stearothermophilus

Two initiation factors have been isolated from the thermophilic bacterium, Bacillus stearothermophilus, and purified to near homogeneity. The two factors possess physical characteristics and activities associated with the E. coli initiation factors IF-2 and IF-3, and are interchangeable with these factors. The two systems present, however, several differences : S-IF-2 is significantly more heat stable than E. coli IF-2, loosing less than 50 per cent of its activity after 20 minutes at 70degreesC. S-IF-2 alone is unable to promote initiation complex formation on B. stearothermophilus or E. coli ribosomes, and S-IF-3 is absolutely necessary for initiation of complex formation on B. stearothermophilus ribosomes. No factor corresponding to IF-1 has been found. S-IF-3 appears to be able to replace at least partially IF-1, since S-IF-3 and E. coli IF-2 are sufficient to promote maximum fMet-tRNA binding to E. coli ribosomes, while E. coli IF-3 and IF-2 also require IF-1. The differences between the two systems are perhaps required because of the elevated temperature at which B. stearothermophilus normally grows.

Antibiotics and compounds affecting tanslation by eukaryotic ribosomes. Specific enhancement of aminoacyl-tRNA binding by methylaxnthines

The mode and site of action of inhibitors of translation (initiation, elongation and termination of protein synthesis) in eukaryotic systems is reviewed. The isolation and characterization of a factor is described that binds Ac-Phe-tRNA to form a complex made up of binding factors, Ac-Phe-tRNA, and ribosome. The binding of Ac-Phe-tRNA probably occurs at the ribosomal site involved in the binding of the initiator substrate Met-tRNAF. The effect of inhibitors of the intitiation phase of protein synthesis on the nonenzymic Ac-Phe-tRNA binding to ribosomes is investigated. The two sites translocation model for translation in eukaryotic cells is presented and the effects of inhibitors on the various steps of protein synthesis are determined empirically. The site of action of inhibitors of peptide bond formation at the ribosomal peptidyl transferase center is elucidated. The action of inhibitors of translocation is sutdied in model cell-free systems from human cells. In addition, a number of methylxanthines are shown to enhance the elongation phase in polypeptide synthesis by stimulating the enzymic binding of aminoacyl-tRNA. The effect of caffeine, theophylline and its derivatives are shown to be fairly specific and dependent on the ribosome concentration. Aminophylline is shown to have a similar effect but also enhances aminoacyl-tRNA synthetase activity at low Mg++ concentrations, probably displacing the optimal concentration of Mg++ in the reaction. This second effect of aminophylline appears to be due to the ethylenediamine moiety of aminophylline since it is also observed in the presence of different polyamines but not in the presence of caffeine or theophylline.

Stimulation by ATP of protein initiation in a prokaryotic organism, B. stearothermophilus

In contrast to E. coli ribosomes, with B. stearothermophilus ribosomes initiation complex formation is stimulated by ATP as well as GTP, but maximum stimulation occurs when both the nucleotides are present; and their terminal phosphate must be hydrolysable. In the presence of ATP and GTP, B. stearothermophilus ribosomes synthesize a highly phosphorylated guanine derivative, ppGpp, and the role of ATP in initiation might be related to this synthesis. We discarded the role of ATP as being trivial and corresponding solely to the well-known effect on eukaryotic systems.