Chromatins of low-protein content: special features of their compaction and condensation.

Histonic chromatin with a relatively high-protein content (RPC of about 1) is compared with naturally occurring chromatins of low-protein contents (RPCs of less than 0.5). The features of these chromatins, with respect to compaction and condensation, are discussed. Liquid crystalline chromatin, as found in dinoflagellates and phage heads, can apparently only be formed by condensation of chromatin of low-protein content and when it is not supercoiled. With histonic chromatin, liquid crystals are never found. Chromatins with low-protein contents might also form compactosomes (or 'labile nucleosomes'), as, for instance, in bacteria. They are forms of supercoiled DNA without a protein core and are so labile that they are difficult to study and even to detect. Chemical fixatives, as commonly used for electron microscopy, do not cross-link the chromatins of low-protein content, a feature which they share with naked DNA. It is postulated that these fixatives even relax the existing supercoil, which seems to be preserved after cryofixation only.