Functional consequences of improved structural information on bacterial nucleoids.

Section of Escherichia coli cells, cryofixed, freeze-substituted into acetone and resin-embedded, show nucleoids with coralline shape. The excrescencies reach far into the cytoplasm. Membrane contact is no longer excluded. Comparison with phase contrast light microscopy shows that the fine excrescencies cannot be resolved and therefore lead "artificially" to a more confined aspect of the nucleoid. The packing density of the DNA in the nucleoids is like that of eukaryotic interphase nuclei and would thus allow diffusion in and out of even large macromolecules. The transcription has, however, been demonstrated to occur only at the periphery; it requires a very dynamic state of the chromatin. The chromatin fine structure is now more granular than fibrillar, as it was previously. The granular structure is compatible with--but there is no proof for--the existence of compactosomes, which would form as a consequence of unrestrained supercoiling.