[Content and properties of different classes of membrane-bound polyribosomes during embryogenesis in chickens]

The quantitative ratio, chemical composition and sedimentation properties of three polyribosomal fractions (free polyribosomes and polyribosomes tightly and loosely bound to the endoplasmic reticulum) isolated from chicken embryos of different age were studied. The fraction of polyribosomes loosely bound to the membranes was obtained from the total preparation of membrane-bound polyribosomes by treatment with high ionic strength solutions. It was found that the polyribosomes loosely bound to the membranes and free polyribosomes possess similar chemical composition and sedimentation properties. All the fractions studied were characterized by a much higher polyribosomal content as compared to the 80S monomers. In the course of embryogenesis the amount of polyribosomes loosely bound to the membranes is decreased, whereas that of tightly bound ones is increased. These data are indicative of an enhanced synthesis of secreted proteins, particularly collagen, on tightly bound polyribosomes during embryogenesis in chicken.