Protein turnover in a moderately halophilic bacterium

Protein turnover was investigated in exponentially growing Vibrio costicola, a moderately halophilic bacterium that can grow in the NaCl concentration range 0.5–3.5 M (with optimal growth at about 1.0 M). In 1.0 or 1.5 M NaCl the breakdown rate of pulse-labelled proteins was about 5%/h whereas in 0.5 M NaCl breakdown was about 9%/h. These results are in contrast to those reported and observed in Escherichia coli which has a turnover rate of 1–2%/h in exponential growth. Growing E. coli in the highest possible NaCl concentration (1.0 M) did not significantly increase protein turnover. Shifting V. costicola from a higher to a lower NaCl concentration increased the rate of turnover of pulse-labelled proteins, whereas shifting it from a lower to a higher NaCl concentration decreased the rate of turnover. The critical factor in these experiments was not the NaCl concentration at which proteins were labelled but that at which the cells were subsequently incubated. The level of breakdown of long-labelled proteins was low (about 2%/h) and was not affected by shifts in NaCl concentration. Breakdown of pulse-labelled protein was inhibited by cyanide and tetracycline but not by iodoacetate, azide. or chloramphenicol. Treatment with streptomycin increased the rate of turnover. Turnover in cell-free systems was lower than in intact cells and was not inhibited by cyanide or tetracycline. It is suggested that the high rate of turnover, even at optimal NaCl concentrations, may reflect errors in protein synthesis, and that the effect of lower NaCl concentrations may be to alter "native" conformation and thus to increase the susceptibility of some of the properly made proteins to proteolysis.