Ribosomal Complexes from an Extremely Halophilic Bacterium and the Role of Cations1

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Concentrated extracts of Halobacterium cutirubrum were prepared at 0 C by gently disrupting cells with a nonionic detergent in a medium containing 3.0 m KCl, 0.5 m NH4Cl, and 0.04 m (or more) magnesium acetate and then treating the gelatinous mass with deoxyribonuclease. On KCl-sucrose gradients containing 0.5 m NH4Cl and 0.04 m magnesium acetate, these extracts showed 30S and 50S ribosomal subunits plus a flat profile of faster-sedimenting material up to high S values. Only after frozen storage or brief incubation of the extract were 70S ribosomes and distinct classes of small polyribosomes detected. Digestion with ribonuclease converted faster-sedimenting material to 70S particles. The presence of chloramphenicol during preparation of the extracts did not affect these results. The evidence suggests that ribosomal particles exist in these cells as subunits or as polyribosomes but not as 70S ribosomes. To investigate the function of Mg++ and NH4+ ions in ribosomal complexes from this halophile, concentrated cell extracts and extracts incubated with 14C-leucine were examined on KCl-sucrose gradients containing different concentrations of these ions. Polyribosomes and the bulk of 70S ribosomes dissociated reversibly to subunits at about 0.01 m Mg++, whereas a small fraction of the 70S particles, including those which in vitro incorporated 14C-leucine into nascent protein, dissociated only below 1 mm Mg++. Below this concentration of Mg++, nascent protein remained attached to the 50S subunit even at 0.04 mm Mg++ in the presence of 0.35 to 0.5 m NH4Cl. Nascent protein, presumably as peptidyl-transfer ribonucleic acid, dissociated reversibly from 50S subunits only at 0.04 mm Mg++ and 0.1 m or less NH4+. Thus, the stability of polyribosomes from H. cutirubrum depends specifically on both Mg++ and NH4+ ions.

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