Synthesis of Exported Proteins by Membrane-Bound Polysomes from Escherichia coli

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Abstract
A membrane-bound fraction of polysomes of Escherichia coli has been isolated after lysis of cells without the use of lysozyme. Protein-synthesis studies in vitro show that membrane-bound and free polysomes are different in the following respects. 1 Membrane-bound polysomes synthesize proteins which are exported from the cell. The products include proteins of the outer membrane and a secreted periplasmic protein, the maltose-binding protein. 2 The major product synthesized by free polysomes is elongation factor Tu, a soluble cyto-plasmic protein. 3 The activity of membrane-bound polysomes in vitro is more resistant to puromycin than is the activity of free polysomes. In addition, the mRNA associated with membrane-bound polysomes is more stable than the bulk of cellular mRNA as revealed by studies with rifampicin.