Origins of molecular biology

This essay is reprinted from the 1988 Annual Report of the Department of Microbiology of the Biozentrum of Basel University with the kind permission of Professor Edward Kellenberger.     

Periplasmic gel: new concept resulting from the reinvestigation of bacterial cell envelope ultrastructure by new methods.

Bacterial cell envelope ultrastructure was investigated both by the progressive lowering of temperature embedding technique and freeze-substitution, using conventional and scanning transmission electron microscopy. Comparison with standard embedding procedures revealed a new aspect of cell envelope structure in specimens at low temperatures. The envelope was delimited by an electron-dark layer, beneath which was a uniform matter-containing layer lying between the outer and inner membranes. There was no empty periplasmic space. Buoyant densities of isolated peptidoglycan obtained in Percoll (1.02 to 1.07 g ml-1) and CsCl2 (1.44 g ml-1) led to a calculated hydration of the peptidoglycan which was more than was previously assumed. Peptidoglycan therefore possibly fills the entire space between the inner and outer membranes in the form of a periplasmic gel. The new model of cell envelope organization is discussed with respect to the current knowledge on bacterial cell wall structure and function.     

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.