CO2 fixation by the facultative autotroph Thiobacillus novellus during autotrophy–heterotrophy interconversions

Abstract

During conversion of Thiobacillus novellus to a heterotrophic mode of growth, 14CO2 incorporation declined in freshly grown cells by about 67% of the autotrophic amount within 15 min after transfer to glucose – mineral salts medium. At later times this activity increased until it was about 350% of the original amount. Enzyme assays of freshly prepared crude extracts revealed that initially RuDP carboxylase accounted for 77 ± 4.7% of the total specific activities of the CO2-fixing enzymes studied. PEP carboxylase was 6.7 ± 1.3%; PEP carboxykinase, 8.2 ± 1.8%; and pyruvate carboxylase, 8.2 ± 1.8%. The decline in specific activity of RuDP carboxylase during growth on glucose was relatively slow, accounting for about 8% of the original activity after 18 h. None was detectable after 24 h growth. Decline in the specific activity of phosphoribulokinase and alkaline fructose diphosphatase was also observed. On the other hand, there was an increase in the specific activities of PEP carboxylase, PEP carboxykinase, and pyruvate carboxylase respectively. Conversion of the organism to an autotrophic mode of growth initially resulted in high incorporation of 14CO2. This, however, declined with increased exposure to S2O32− and CO2. The specific activities of the various enzymes also showed a reversal of the patterns noted during heterotrophic development. Phosphoribulokinase reached its maximum (specific activity) earliest. It was followed about 2 h later by RuDP carboxylase, and another 2 h elapsed before alkaline fructose diphosphatase attained its maximum. Finally, S2O32− was found to be necessary for RuDP carboxylase induction; HCO3−, SO32−, or SO32− and HCO3− were ineffective.