Molecular Control and Biochemistry of CO₂ Fixation in Photosynthetic Bacteria

Abstract

Photosynthetic bacteria are capable of diverse modes of CO₂ assimilation, employing either the Calvin reductive pentose phosphate, reductive tricarboxylic acid, or hydroxyproprionate cycles, or in some cases various ancillary reactions. In purple bacteria and cyanobacteria, the Calvin reductive pentose phosphate pathway predominates (Tabita 1988, 1994, 1995), while green sulfur bacteria use the reductive TCA cycle and green nonsulfur bacteria employ the hydroxypropionate path (Sirevåg 1995). Biochemical studies in our laboratory have focused on the enzymology of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) and phosphoribulokinase (PRK), the key enzymes of the Calvin cycle, and ATP-citrate lyase and the ferrodoxin-linked reactions of the reductive TCA cycle. The molecular control of CO₂ fixation in nonsulfur purple bacteria has been a continuous interest (Gibson 1995), while the organism Chlorobium tepidum (Wahlund et al. 1991) has recently provided us with a useful and genetically tractable model system to study regulation of the reductive TCA cycle.