Salt-sensitive in vitro protein synthesis by a moderately halophilic bacterium

Abstract

EXTREMELY halophilic bacteria, such as Halobacterium cutirubrum, grow only in high NaCl concentrations (2.5–5 M) and have very high (5 M or higher) internal concentrations of salts, mainly KCl^1,2. The ribosomes and protein-synthesising systems of these organisms seem especially adapted to function in such concentrations³. In contrast, moderately halophilic bacteria, such as Vibrio costicola, grow over a much wider range of NaCl concentrations (at least 0.5–3.5 M)⁴. In V. costicola, though not necessarily in all moderate halophiles, the cell-associated monovalent cations are at least as concentrated as those of the external medium. For example, cells growing exponentially in medium containing 1 M NaCl and 0.008 M KC1 can have internal Na⁺, K⁺, and NH₄⁺ concentrations of about 0.6, 0.7 and 0.4 M respectively, as well as 40 mM Mg²⁺ (ref. 5 and our unpublished results). Ribosomes from V. costicola differ from those of both extremely halophilic and non-halophilic bacteria in their ability to maintain a ‘standard’ sedimentation pattern (30, 50 and 70S) over a wide range of salt concentrations. This pattern is not changed by the NaCl concentration in which the cells are grown⁶. Such properties, however, do not really tell us how well the ribosomes function at different salt concentrations. Studies of in vitro protein synthesis, reported here, suggest that ribosomes may function at much lower salt concentrations than measurements of total cell-associated ions indicated to be present in the cell.