Chromosomal structure of the halophilic archaebacterium Halobacterium salinarium

Abstract

The chromosomal structure of the extremely halophilic archaebacterium Halobacterium salinarium was examined. Sheared chromosomes prepared from the bacteria in the late exponential phase were separated into two peaks (peaks I and H) by sucrose gradient centrifugation, suggesting that the chromosomes consist of two parts differing in quality. The UV spectra of peaks I and H resembled those of DNA and eukaryotic chromatin, respectively. Electron microscopic observations revealed that the major component of peak I was protein-free DNA, while the major components of peak II were rugged thick fibers with a diameter of 17 to 20 nm. The rugged fibers basically consisted of bacterial nucleosome-like structures composed of DNA and protein, as demonstrated in experiments with proteinase and nuclease digestion. Whole-mount electron microscopic observations of the chromosomes directly spread onto a water surface revealed a configuration in which the above-described regions were localized on a continuous DNA fiber. From these results it is concluded that the H. salinarium chromosome is composed of regions of protein-free DNA and DNA associated with nucleosomelike structures. Peaks I and H were predominant in the early exponential phase and stationary phase, respectively; therefore, the transition of the chromosomal structure between non-protein-associated and protein-associated forms seems to be related to the bacterial growth phase.

Nucleoid structure and distribution in thermophilic Archaea

“…The high degree of structural organization described here is observed also in the chromosome DNA of organisms from the other prokaryotic branch of the evolutionary tree, the Bacteria domain (reviewed in reference 17). Still, archaeal chromosome DNA is histone associated and organized into nucleosome-like structures (9,21), and the replication-associated genes that have been detected in the M. jannaschii genome are generally similar to eukaryotic genes (7). The separation of the nucleoids to opposite cell halves before division is reminiscent of a recent model for nucleoid partition in eubacteria (reviewed in reference 23), in which replication origins become attached to the cell poles before division.…”

Sulfolobus hakonensis sp. nov., a Novel Species of Acidothermophilic Archaeon

“…Tris-HC1 (pH 7.5) and centrifuged at 10,000 X g for 1 min and then spread onto a water surface, as described previously (28). Intact or broken cells, cell membrane fragments, and cell ghosts floating on the surface were picked up with carbon-coated grids and stained with 1.0% uranyl acetate; they were then observed by electron microscopy directly or after rotary shadowing with Pt-Pd (4:l) at an angle of tan' (l/lO), as described by Marmur (19).…”

Archaeal chromatin: Virtual or real?

“…As mentioned above, EM images of spreads of DNA fibers revealed NLS in T. acidophilum (10) and H. salinarium (12,13). The work of Pereira et al (9) adds M. thermoautotrophicum to this list.…”