Cytoplasmic methionine transfer RNAs from eukaryotes

Synthesis of and chains of rabbit hemoglobin in a cell-free extract from Krebs II ascites cells

An RNA sedimenting at 10 S, with a molecular weight of 2.3 x 10(5), was isolated from rabbit reticylocyte polyribosomes. When this RNA is added to a cell-free extract from Krebs II ascites cells, both alpha and beta chains of rabbit hemoglobin are synthesized; beta chains are made in about 50% excess over alpha chains.

Protein synthesis directed by encephalomyocarditis virus RNA: properties of a transfer RNA-dependent system

Small amounts of encephalomyocarditis virus RNA direct a 50-fold increase in amino acid incorporation, in appropriately supplemented ascites tumor cell extracts, under conditions that give rise to authentic viral polypeptides. Incorporation in these crude extracts has a novel characteristic, namely, that it is almost entirely dependent upon the addition of exogenous tRNA. Further, this incorporation is restricted in that tRNA derived from ascites tumor cells or from rat liver permits translation of viral RNA, whereas tRNA from yeast or Escherichia coli does not. These translational barriers are due, at least in part, to an incompatibility between the tRNA of yeast and E. coli and the aminoacyl-tRNA synthetases of the ascites tumor cell. A more extensive basis for this incompatibility is suggested, however, by the failure of the E. coli aminoacyl-tRNA synthetases to restore viral RNA-directed protein synthesis in the presence of tRNA from E. coli, although the coli synthetases fully restore the poly(U)-directed synthesis of polyphenyl-alanine. The possible role that unique or favored codon classes might play in this restriction is considered, together with the implications of the observed requirement for tRNA.

Requirement for GTP in the initiation process on reticulocyte ribosomes and ribosomal subunits

The requirement for GTP in the initiation process on reticulocyte ribosomes and ribosomal subunits has been examined by studying Met-tRNA(F) binding, ribosome-dependent [gamma-(32)P]GTP hydrolysis, and peptide-bond formation with puromycin. Met-tRNA(F) binding can be obtained with the methylene analogue, 5'-guanylylmethylene diphosphonate, as well as GTP, and it is not inhibited by fusidic acid or several other inhibitors of protein synthesis. This reaction can be performed with the 40S subunit and has the same requirements as the Met-tRNA(F)-binding reaction with washed ribosomes. Ribosome-dependent [gamma-(32)P]GTP hydrolysis can be obtained with the initiation factor M(2A) using either washed ribosomes or the 40S subunit. This reaction is also not significantly inhibited by fusidic acid. Peptide-bond formation between puromycin and Met-tRNA(F), however, is inhibited by fusidic acid, and does not occur if the methylene analogue of GTP is substituted for GTP. These data suggest that the binding of the initiator tRNA to the 40S subunit does not require the hydrolysis of GTP, but that at least one GTP hydrolysis event must occur after Met-tRNA(F) binding in order for the first peptide bond to be formed.

NH 2 -terminal residues of Neurospora crassa proteins

The NH(2)-terminal amino acid composition of the soluble and ribosomal proteins from Neurospora crassa mycelia and conidia was determined by the dinitrophenyl method. A nonrandom distribution of NH(2)-terminal amino acids was observed in the complex protein mixtures. Glycine, alanine, and serine accounted for 75% of the NH(2)-terminal amino acids, and glycine appeared most frequently in mature proteins of mycelia. The appearance of phenylalanine as one of the major NH(2)-termini in crude conidial fraction suggests that the composition of proteins may vary in different developmental stages.

Interaction of eukaryote initiator methionyl-tRNA with the eukaryote equivalent of bacterial elongation factor T and guanosine triphosphate

The initiator tRNA, methionyl-tRNA(i) (Met), of yeast and wheat germ forms relatively unstable ternary complexes with their corresponding elongation factors T and GTP. Such complexes can be demonstrated only with fast separation techniques such as Sephadex G-50 and Millipore filtration, but not with the slow Sephadex G-100 method, although both techniques yield stable ternary complexes with all other aminoacyl-tRNAs, including the internal Met-tRNA(m) (Met). To bind yeast-initiating Met-tRNA(i) (Met) to ribosomes, initiation factors present in a ribosomal wash fraction from yeast are needed.

Initial dipeptide formation in hemoglobin biosynthesis

Initiation factors M(1) + M(2) from reticulocyte ribosomes bind Met-tRNA(F) to rabbit reticulocyte ribosomes containing endogenous hemoglobin mRNA. The initial binding of Met-tRNA(F) appears to be to the small ribosomal subunit. The Met-tRNA(F) is able to participate in what is presumed to be the first peptide bond in the formation of hemoglobin, namely the synthesis of a methionyl-valine dipeptide. The formation of this methionyl-valine dipeptide requires Met-tRNA(F), initiation factors M(1), M(2), and M(3), as well as Val-tRNA and T(1). No synthesis of methionyl-valine dipeptide takes place if Met-tRNA(F) is replaced by Met-tRNA(M), or if initiation factor M(3) is omitted. Thus, Met-tRNA(F) appears to be the initiator tRNA for hemoglobin biosynthesis and M(3), although required for the synthesis of the first peptide bond of hemoglobin, does not appear to be necessary, under the experimental conditions studied, for Met-tRNA(F) binding.

Mammalian peptide chain termination. II. Codon specificity and GTPase activity of release factor

In vitro peptide chain termination with release factor preparations from rabbit reticulocytes, guinea pig liver, or Chinese hamster liver is directed with UAAA, UAGA, or UGAA, suggesting that UAA, UAG, and UGA are terminator condons for mammalian cells. Purified release factor from rabbit reticulocytes has ribosomaldependent GTPase activity, which is stimulated by UAAA. GTP hydrolysis appears requisite for in vitro peptide chain termination in mammals.

Specific transformylation of one methionyl-tRNA from cotton seeding chloroplasts by endogenous and Escherichia coli transformylases

Two isoaccepting chloroplastic and one cytoplasmic tRNA(Met) species have been separated from germinating cotton cotyledons. The methionylated form of one of the chloroplastic species (but none of the other or of the cytoplasmic tRNA(Met)) can be formylated either by an endogenous transformylase or by Escherichia coli transformylase.

Puromycin-peptide bond formation with reticulocyte initiation factors M1 and M2

The ability to form a "peptide" bond between various forms of Met-tRNA or Phe-tRNA and puromycin has been studied in the reticulocyte cell-free system. When Met-tRNA(F), fMet-tRNA(F), or N-acetylPhe-tRNA are used as substrate at low Mg(++) concentration (3 mM), reticulocyte initiation factors M(1) and M(2) (M(2A) + M(2B)) are required for puromycin-peptide synthesis. In contrast to bacterial systems, this reaction is also stimulated by the elongation factor T(1). When Met-tRNA(M) or Phe-tRNA is used as substrate, there is no M-factor requirement for the puromycin reaction; T(1) is absolutely required, and the reaction is stimulated by T(2). These studies indicate that reticulocyte factors M(1) and M(2) may function in part by placing the initiator tRNA into the P site. The detailed mechanism for mammalian initiation, however, may be more complex than that for bacterial systems.

Isolation of Saccharomyces cerevisiae mitochondrial formyltetrahydrofolic acid:methionyl-tRNA transformylase and the hybridization of mitochondrial fMet-tRNA with mitochondrial DNA

Formyltetrahydrofolic acid:methionyl-tRNA transformylase was isolated from Saccharomyces cerevisiae mitochondria and used to prepare yeast mitochondrial [(3)H]formylmethionyl-tRNA. This fMet-tRNA hybridizes with mitochondrial DNA but not with yeast nuclear or E. coli DNA. Unlabeled mitochondrial, but not extramitochondrial, tRNA competes in this reaction. tRNA was eluted from the hybrid and found to contain the label almost exclusively in fMet-tRNA. Yeast cytoplasmic fMet-tRNA formylated with Escherichia coli enzyme, and E. coli fMet-tRNA, do not hybridize with mitochondrial DNA. It is concluded that yeast mitochondrial tRNA(fMet) is a gene product of the mitochondrial genome.

Protein chain initiation by methionyl-tRNA in wheat embryo

The mechanism of protein chain initiation has been investigated in a cell-free amino acid incorporation system from wheat embryos dependent on tobacco mosaic virus RNA. Analysis of the N-termini of the labeled peptide products of short-term incubations showed the presence of unblocked methionine. In addition, methionyl-tRNA (Met-tRNA) could be bound to ribosomes at 1.3 mM Mg(++) in a reaction requiring viral RNA, ATP, GTP, and soluble protein factors. Incorporation experiments with the two cytoplasmic Met-tRNAs of wheat germ, an initiating species designated Met-tRNA(i) and a Met-tRNA(m), showed that methionine transfer from Met-tRNA(i) was linear from zero time, while that from Met-tRNA(m) occurred only after an appreciable lag. Analysis of the peptide products showed that methionine transfer from Met-tRNA(i) was predominantly N-terminal. In contrast, methionine transfer from Met-tRNA(m) was exclusively into internal positions. Similar selectivity was observed in the ribosome binding assay; only Met-tRNA(i) showed a strong reaction. These experiments provide strong evidence that in the wheat embryo, cytoplasmic Met-tRNA(i) functions without formylation in the initiation of protein synthesis.

Seryl-tRNA in mammalian tissues: chromatographic differences in brain and liver and a specific response to the codon, UGA

Differences in the elution profiles of seryl-tRNAs of beef and of rabbit liver and brain were observed. A species of beef seryl-tRNA obtained by reversed phase column chromatography that responds specifically to the codon UGA was found. In addition, a species of rabbit and of chicken seryl-tRNA that recognizes UGA was found.