Factors Affecting the Synthesis and Degradation of Ribulose-1,5-Diphosphate Carboxylase in Hydrogenomonas facilis and Hydrogenomonas eutropha

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Hydrogenomonas facilis and H. eutropha cultured in fructose medium retained high levels of ribulose-1,5-diphosphate carboxylase only when the following conditions were fulfilled: low aeration, FeCl3 addition to fructose medium, and cell harvest at or prior to mid-exponential phase of growth. Repression of carboxylase synthesis was demonstrated under conditions of high oxygen tension during growth of H. eutropha on fructose. Upon depletion of fructose in the growth medium, carboxylase activity fell abruptly in both organisms. The decline could not be attributed to a repressive mechanism. Rapid inactivation of carboxylase was promoted by transfer of mid-exponential-phase H. eutropha to a basal salts medium lacking fructose. During severe fructose starvation, N2, H2, 80% H2 to 20% air, 2,4-dinitrophenol, actinomycin D, streptomycin, bicarbonate, and magnesium ion deficiency spared carboxylase. Nitrogen starvation or chloramphenicol afforded no protection during severe starvation. In vitro inactivation was also demonstrated in crude cell-free extracts from nonstarved, fructose-grown H. eutropha. Substrate bicarbonate protected against this loss. Inactivation of the carboxylase could not be demonstrated either by starvation of autotrophically grown cells or in autotrophic extracts. Autotrophic extracts mixed with heterotrophic extracts lost their carboxylase activity, but mixing with heterotrophic extracts that had been heated to 50 C resulted in no loss of activity. Mechanisms are proposed to accommodate these observations.

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