Mechanism of protein biosynthesis

Identification of the cytokinin-active ribonucleosides in pure Escherichia coli tRNA species

The distribution of the two cytokinin-active ribonucleosides 6-(3-methyl-2-butenylamino)-9-beta-D-ribofuranosylpurine (1) and 6-(3-methyl-2-butenylamino)-2-methylthio-9-beta-D-ribofuranosylpurine (2) in tRNA(Phe), tRNA(UUG) (Leu), tRNA(UCR) (Ser), tRNA(Tyr), tRNA(Cys), and tRNA(2) (Trp) from stationary phase Escherichia coli has been studied. Compound 2 predominated in all these species except tRNA(Phe), in which compound 1 was greatly in excess. Compound 1 was present in relatively small amounts in tRNA(UUG) (Leu), tRNA(Cys), and tRNA(2) (Trp), and was not detected in tRNA(UCR) (Ser) and tRNA(Tyr). Cells from the early logarithmic phase had the highest cytokinin content in their tRNA. Stationary phase tRNA had a slight increase in the amount of 1, which may be explained by the appearance of the new tRNA(Phe). No differences were observed between the chromatographic patterns of tRNAs from early-, mid-, and latelogarithmic phases.

Inhibitory effect of diphtheria toxin on amino acid incorporation in Escherichia coli cell-free system

The mechanism of action of diphtheria toxin in an Escherichia coli cell-free protein-synthesizing system was examined. When the washed ribosomes were incubated with toxin before addition of messenger ribonucleic acid (RNA), peptide syntheses of (14)C-phenylalanine directed by polyuridylic acid or phage R17 RNA were strongly inhibited by a small amount of toxin. Whereas, if the soluble fraction (105,000 x g supernatant fraction) was preincubated with toxin, no effect of toxin occurred either on the induced protein synthesis or on the activity of guanosine triphosphatase even in the presence of nicotinamide adenine dinucleotide. The binding of (3)H-formylmethionyl-transfer RNA to E. coli ribosomes directed by either R17 RNA or trinucleotide AUG was also decreased by toxin. These findings suggest that diphtheria toxin may prevent the binding of messenger RNA by successfully competing with the AUG for ribosomal binding sites. Sucrose-density gradient studies support this concept by showing the decrease in binding of (3)H-labeled R17 RNA to E. coli ribosomes exposed to toxin.

Peptide chain termination with mammalian release factor

We report a method for the in vitro study of peptide chain termination in mammals. A proteinaceous release factor has been isolated from rabbit reticulocyte extracts. This factor and a polyribonucleotide template containing the bases U and A stimulate the release of N-formylmethionine from mammalian [N-formylmethionyl-tRNA(f).ribosome] intermediates. Our studies suggest that UAA is a terminator codon for mammalian cells.

Mutants of Escherichia coli K-12 with an altered glutamyl-transfer ribonucleic acid synthetase

Three streptomycin-suppressible lethal mutants of Escherichia coli K-12 have been shown to possess structurally altered glutamyl-transfer ribonucleic acid (tRNA) synthetases. Each mutant synthetase displays a K(m) value for glutamate which is 10-fold higher than the parental value, and the mutations reside in two widely separate loci on the genetic map. Mixing of the mutant extracts in pairs gave no indication of in vitro complementation. All three enzymes charge the minor tRNA(glu) fraction identically, but one (EM 120) charges the major fraction at a twofold lower rate than do the other two (EM 102 and EM 111). Possible explanations for the existence of the two synthetase loci are presented.

Binding of adenosine 3':5'-cyclic phosphate to G factor of Escherichia coli, and its effects on GTPase, RNase V, and protein synthesis

Unique among adenine nucleotides tested by filter binding assays, 3':5'-cyclic AMP binds to the G translocation factor. Binding is dependent on the presence of GTP, and is inhibited by GDP, by the analog 5'-beta,gamma-methylene GTP, and by the antibiotic fusidic acid. The cAMP seems to be released during the ribosome-dependent translocation of charged tRNA catalyzed by G factor. Bound cAMP inhibits GTPase and ribosome-associated degradation of messenger RNA, but does not inhibit protein synthesis. cAMP might thereby regulate the ratio of productive to degradative transits of ribosomes on messenger RNA, and this may account for some part of its profound effect on levels of specific bacterial messenger RNA species.

Studies on the mode of action of diphtheria toxin. III. Effect on subcellular components of protein synthesis from the tissues of intoxicated guinea pigs and rats

The effect of diphtheria toxin on subcellular components of protein synthesis was determined. Polyribosomes prepared from intoxicated guinea pigs functioned normally in an in vitro assay system, while the activity of soluble enzymes (transferases) from toxin-treated animals was significantly reduced. At high toxin dosages, this reduction was widespread, but when levels of toxin comparable to those which might be generated in a natural infection were given, inhibition of soluble enzyme activity was found only in extracts from heart and skeletal muscle. Possible nonspecific inhibition in the assay system due to interference by free toxin or by a serum component was eliminated. Since it was possible to demonstrate reactivation of soluble enzyme activity with nicotinamide and toxin, it was suggested that diphtheria toxin acts in the intact sensitive animal in a manner analogous to its action in tissue culture or in cell-free systems. It was hypothesized that the lethal biochemical lesion of the toxin in sensitive animals was the inactivation of transferase enzymes, principally in the heart. It was also suggested that the lethal lesion induced in diphtheria-sensitive and resistant species may not be identical.

Peptide chain termination. VI. Purification and site of action of S

PEPTIDE CHAIN TERMINATION IS A RESULT OF AT LEAST TWO EVENTS: terminator codon recognition and hydrolysis of peptidyl tRNA. A protein factor S, isolated from the supernatant of Escherichia coli B, stimulates fMet release. Factor S lowers the K(m) for terminator trinucleotides without altering the V(max) of release and therefore acts at terminator codon recognition. The S protein differs from initiation factors, elongation factor G, several forms of elongation factor T and release factors. The importance of the 2% Tu content in purified S is unresolved.

Ribonuclease V of escherichia coli. I. Dependence on ribosomes and translocation

A new RNase activity, tentatively named RNase V, was found in cell-free extracts of E. coli. This activity requires ribosomes, G and T factors, tRNA, K(+) or NH(4) (+), Mg(2+), GTP, and a sulfhydryl compound to degrade poly U, poly A, T4 phage mRNA, or E. coli mRNA. RNase V is specific for mRNA; it does not attack ribosomal RNA. It is inhibited by antibiotics that decrease breakdown of mRNA in vivo, such as chloramphenicol and streptomycin, and by such agents as 5'-beta, gamma-methylene-guanosine triphosphate, and fusidic acid, which inhibit ribosome-dependent GTPase and translocation of ribosomes along mRNA. The evidence suggests that RNase V is either an integral part of the ribosome or is tightly associated with it, and that it selectively degrades mRNA in intact cells.