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I. This paper describes some features of the protein synthesis in isolated rat liver mitochondria and reports on the stability of the product of mitochondrial protein synthesis in vitro. 2. Chase experiments performed after a 30-min pulse of [14C]leucine indicated that a large part of the product is degraded to an acid-soluble form (labile product) almost as rapidly as it is terminated and released from the mitoribosome. Only a small proportion of the completed product (between 20 and 40 %)is conversed in acid-insoluble form (stable product). 2. Both labile and stable products have an exceptional degree of hydrophobicity, shown by their marked resistance to acid and alkaline hydrolysis and by their solubility in organic solvents. 4. A 14C-labelled peptide with an apparent molecular weight of 12000-13000, as assessed by gel electrophoresis in the presence of dodecylsulfate, is released after heat treatment of mitoribosomes isolated from mitochondria pulsed with [14C]leucine. [ABSTRACT FROM AUTHOR]

In the accompanying paper, we present evidence to show that the biosynthesis of the two characteristic polypeptide constituents ‘A’ and ‘B’ of the crystal protein toxin of Bacillus thuringiensis car. tolworth, is restricted to a short interval, from 16–18 h, in the mid-stationary phase of the growth cycle of 24 h in culture. Here we examine the question of the kinetics of biosynthesis of the corresponding crystal protein messenger RNA. We have taken the direct, approach of assaying a series of RNAs isolated from B. thuringiensis cultures at varying times of culture for messenger activity in a heterologous Escherichia coli, subcelular system. The product of RNA-stimulated protein synthesis in vitro was characterized by immunoprecipitation with anti-serum against purified crystal protein and by polyacrylamide-gel electrophoresis of the antigen-antibody precipitates dissolved in the detergent sodium dodecylsulfate. RNA from B. thuringiensis, but not that of Bacillus cerens, stimulates the synthesis of protein which specifically precipitates with crystal protein antiserum. The time of extraction of RNA from B. thurinrjgiensis does not appear to be critical in determining its messenger activity : all samples taken in the interval of 11–20 h contained messenger RNA for crystal protein by the criterion of specific immunoprecipitation. In each case a significant proportion of the synthesized protein migrated in the region of the smaller-molecular-weight component A (55000) of the crystal protein in the electrophoretic separation. However, hardly any synthesized protein co-migrated with the larger-molecular-weight component B (120000) of this protein. Since all the labelled protein precipitated by the crystal protein antiserum was larger than about 15000 molecular weight we conclude that mainly large fragments of the crystal protein were synthesized in the E. coli system. We failed in our attempts to isolate polysomes as a source of messenger RNA from stationary-phase ceils. Thus it was not possible to determine by experiments similar to those described above whether the crystal protein messenge is stored in ribosomes or in some other fraction of the cell when it is not being translated. [ABSTRACT FROM AUTHOR]