1. pH5 enzyme from non-lactating bovine mammary gland was found to contain potent inhibitors of protein synthesis in the rat liver cell-free system. These inhibitors affect (a) formation of aminoacyl-tRNA where tRNA represents transfer RNA, (b) transfer of labelled amino acids from rat liver amino[(14)C]acyl-tRNA to protein in rat liver polyribosomes, and (c) incorporation of (14)C-labelled amino acids into peptide by rat liver polyribosomes supplemented with rat liver pH5 enzyme. 2. Increasing amounts of pH5 enzyme from bovine mammary gland progressively inhibited the incorporation of labelled amino acids into protein by a complete incorporating system from rat liver. Approx. 80% inhibition was observed at a concentration of 2mg. of protein of pH5 enzyme from bovine mammary gland. The inhibitory effect of the bovine pH5 enzyme fraction could not be overcome by the addition of increasing amounts of rat liver pH5 enzyme. 3. Fractionation of bovine pH5 enzyme with ammonium sulphate into four fractions showed that all the fractions inhibited the incorporation of (14)C-labelled amino acids in the rat liver system, but to varying extents. The highest inhibition observed (90%) was exhibited by the 60%-saturated-ammonium sulphate fraction. 4. Heat treatment of bovine pH5 enzyme at various temperatures caused only a partial loss of its inhibitory effect on labelled amino acid incorporation by the rat liver system. Treatment at 105 degrees for 5min. resulted in the bovine pH5 enzyme fraction losing 30% of its inhibitory activity. 5. pH5 enzyme from bovine mammary gland strongly inhibited the charging of rat liver tRNA in the presence of its own pH5 enzymes. 6. The transfer of labelled amino acids from rat liver amino[(14)C]acyl-tRNA to protein in a system containing rat liver polyribosomes and pH5 enzyme was almost completely inhibited by bovine pH5 enzyme at a concentration of 2mg. of protein of the enzyme fraction. 7. One of the inhibitors of various stages of protein synthesis in rat liver present in bovine pH5 enzyme was identified as an active ribonuclease, and the second inhibitor present was shown to be tRNA.

1. tRNA isolated from non-lactating bovine mammary gland competitively inhibits the formation of aminoacyl-tRNA in the rat liver system. 2. Non-lactating bovine mammary gland tRNA and twice-pyrophosphorolysed rat liver tRNA are unable to accept amino acids in a reaction catalysed by aminoacyl-tRNA synthetases from either rat liver or bovine mammary gland. Deacylated rat liver tRNA can however be aminoacylated in the presence of either enzyme. 3. Bovine mammary gland tRNA lacks the terminal adenine nucleotide at the 3'-terminus amino acid acceptor end, which can be replaced by incubation in the presence of rat liver nucleotide-incorporating enzyme, ATP and CTP. 4. The enzymically modified bovine tRNA (tRNApCpCpA) can bind labelled amino acids to form aminoacyl-tRNA, which can then transfer its labelled amino acids to growing polypeptide chains on ribosomes. 5. Molecules of rat liver tRNA or bovine mammary gland tRNA that lack the terminal adenine nucleotide or the terminal cytosine and adenine nucleotides inhibit the aminoacylation of normal rat liver tRNA to varying degrees. tRNA molecules lacking the terminal -pCpCpA nucleotide sequence exhibit the major inhibitory effect. 6. The enzyme fraction from bovine mammary gland corresponding to that containing the nucleotide-incorporating enzyme in rat liver is unable to catalyse the incorporation of cytosine and adenine nucleotides in pyrophosphorolysed rat liver tRNA and deacylated bovine tRNA. This fraction also markedly inhibits the action of the rat liver nucleotide-incorporating enzyme.

1. The enzyme fraction obtained from the pH5 enzyme of non-lactating bovine mammary gland between 40 and 100% ammonium sulphate saturation markedly inhibited the AMP-incorporating activity of rat liver nucleotide-incorporating enzyme. This inhibitory effect has been attributed to high nuclease activity which can be partially removed by adsorption of the enzyme fraction on to calcium phosphate gel. 2. The degradation action of the calcium phosphate-purified enzyme is confined mainly to the terminal trinucleotide sequence -pCpCpA of tRNA, its effect being analogous to that of venom phosphodiesterase. This enzyme is heat labile and very readily loses its degradative activity. 3. Treatment of the enzyme fraction with Macaloid results in complete removal of the phosphodiesterase, leaving an enzyme capable of incorporating AMP into tRNA. 4. Transfer RNA extracted from non-lactating bovine mammary gland in the presence of polyvinyl sulphate and Macaloid is able to accept amino acids with an efficiency 30% of that shown by lactating bovine mammary-gland tRNA isolated under identical conditions.

1. Ribosomes prepared from bovine lactating mammary gland are able to synthesize protein, whereas similar preparations from non-lactating glands are not. Washing the ribosome suspensions through a medium containing 0.5m-ammonium chloride enhanced their ability to incorporate phenylalanine into polyphenylalanine. 2. Ribosomes isolated from non-lactating bovine mammary gland, in contrast with those from rat liver and lactating mammary gland, contained significant amounts of extraneous nucleases. These enzymes could be removed by washing with a medium A buffer containing 0.5m-ammonium chloride. 3. Only those ribosomes from functionally active tissues were able to bind polyuridylic acid and phenylalanyl-tRNA.

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