Use of Purified Polysomes from Rabbit Reticulocytes in a Specific

Initiation factors, extracted from ribo- somal particles by high salt, are usually tested for with the extracted ribosomes. However, it is theoretically possible that such tests may respond not only to factors (i.e., pro- teins that join and leave a ribosome during its cycle), but also to true ribosomal proteins that may have been ex- tracted. We have eliminated this circularity by tests with purified polysomres, which have not been extracted with high salt; the polysomes are deficient in initiation factors because they have been largely separated from the native subunits. Tests with both systems, applied to extracts of different classes of particles, suggest that high salt does extract both initiation factors and essential ribosomal proteins. Extracts of native subunits contain both, and stimulate both systems; while, in the presence of these extracts, the extracts of polysomes further stimulate only the extracted ribosomes.