Protein-depleted chromosomes. II. Experiments concerning the reality of chromosome scaffolds.

Chromosome scaffolds are chromosome-shaped bodies, composed of non-histone proteins, which remain when the histones are extracted from chromosomes. Because of the well-known tendency of chromosomal proteins to aggregate, we have tested the possibility that chromosome scaffolds might be produced by aggregation of proteins during the preparation of scaffolds. Extraction of histones in the presence of sucrose, which inhibits aggregation, results in a much looser structure lacking the characteristic appearance of a scaffold, although sucrose does not extract any extra proteins. Extraction of histones from chromosomes in situ on EM grids produced only a network of fine fibres without a scaffold-like structure, while digestion of DNA from typical chromosome scaffolds in situ fell only discrete particles of protein and not a continuous structure. We conclude, therefore, that the typical appearance of chromosome scaffolds produced by histone extraction may well represent an artefact resulting from protein aggregation. Our experiments suggest further that DNA, as well as protein, is a structural component of whatever type of core structure is responsible for maintaining the form of chromosome.