CHROMOSOMAL STRUCTURE OF THE HALOPHILIC ARCHAEBACTERIUM HALOBACTERIUM-SALINARIUM

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 II) by sucrose gradient centrifugation, suggesting that the chromosomes consist of two parts differing in quality. The UV spectra of peaks I and II 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 nucleosome-like structures. Peaks I and II 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.