About the organisation of condensed and decondensed non-eukaryotic DNA and the concept of vegetative DNA (a critical review)

Abstract

Experiments are reviewed that allow one to assign naturally occurring DNA-containing plasmas to either of two classes by virtue of their sensitivity to aggregation upon dehydration in organic solvents. The interphase nuclei of higher cells are relatively insensitive, while the DNA plasmas represented by bacterial nucleoids, vegetative bacteriophage and the chromosomes of dinoflagellates are sensitive. In higher cells the bulk of DNA is organised with histones in the form of nucleosomes. In prokaryotes and in the pool of vegetative phage DNA the most abundant histone-like protein HU is not associated with the bulk DNA, but localised in the border region with ribosomes where transcription and translation occur. These experimental results strongly suggest that the two classes of DNA plasmas are distinguishable by a low (1:10) or high (1:1) protein-to-DNA ratio. The hypothesis is formulated that the vegetative DNA (replicating and transcribing), throughout the living world, is nucleosome-free; during evolution, nucleosomes would have been introduced as a simple and adequate means for compacting the resting DNA. Condensation of DNA does not occur with prokaryotic nucleoids, but does take place when DNA is withdrawn from the vegetative phage pool to become packaged into phage heads. Dinoflagellate chromosomes are rather condensed although structurally different from eukaryotic chromosomes (e.g., those from Euglena) and are much more aggregation-sensitive.