Electron microscopy of polyribosomes synthesizing immunoglobulin chains. Comparison with sizes determined by density-gradient-sedimentation data.

Polyribosomes from the immunoglobulin-producing plasma-cell tumour X5563 were fractionated on a linear sucrose gradient and fractions containing polyribosomes carrying heavy (H) and light (L) immunoglobulin chains were examined with the electron microscope. The size distribution of the polyribosomes in the various fractions was determined. The results are consistent with previous estimates by Williamson & Askonas (1967) of the size of the polyribosomes synthesizing immunoglobulin chains, despite some discrepancies in their calculations based on sedimentation data. In particular, polyribosomes of up to 18 ribosomes were present in the part of the gradient containing H-chain polyribosomes. The polyribosomal clusters appeared to be true linear aggregates of uniformly spaced ribosomes, arranged mostly in open or zig-zag configuration. The appearance of most of them does not seem to support a general helical model of the polyribosome.