[Functional and structural characteristics of polyribosomes associated with chick embryo cell nuclei]

Polyribosomes bound to the outer nuclear membrane was isolated from purified preparations of chicken embryo cell nuclei. These polyribosomes were shown to consist fractions forming unstable complexes with the nuclear membrane which can be separated from the latter by treatment with high ionic strength buffer solutions. Using sedimentation and gradient density analyses, the nuclei-bound RNP complexes were shown to be predominantly composed of 80S monosomes which take an active part in collagen polypeptide synthesis in cell-free protein-synthesizing systems. A comparison of sedimentation properties and collagen-synthesizing activity of nuclei-bound polyribosomes and cytoplasmic polyribosomes forming unstable complexes with endoplasmic membranes, it was concluded that the nuclei-bound 80S monosomes are an early step in the formation of cytoplasmic polyribosomes.