The site of protein synthesis in Bacillus megaterium

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Abstract
The incorporation of C14-labeled amino acids into protein was studied in whole cells, in protoplasts, and in various subcellular fractions derived from B. megaterium. The cell wall does not incorporate any of the amino acids studied into its "protein" to any great extent, but it absorbs a large quantity of free amino acid. Initially, labeled amino acids are incorporated most rapidly into the protein derived from the cytoplasmic membranes of whole cells and protoplasts incubated in growth media. At later times the cytoplasmic proteins attain higher specific activities. At all times the level of the non-protein radioactivity is highest in the cytoplasm. Isolated cytoplasmic membranes can incorporate C14 lab-eled amino acids into their protein fractions at rates comparable with those observed in the membranes of whole protoplasts: the incorporation is linear for at least 2 hours. In comparison, isolated cytoplasm incorporated very small amounts of amino acid into protein. If membranes labeled in this way are incubated with non-labeled cytoplasm, a considerable proportion of the radioactivity is rapidly transferred from the membrane protein to the cytoplasmic protein. It is concluded that the initial stages of protein synthesis in B. megaterium take place at sites on or closely associated with the cytoplasmic membrane.