Biological carbon dioxide fixation is an essential and crucial process catalyzed by both prokaryotic and eukaryotic organisms to allow ubiquitous atmospheric CO2to be reduced to usable forms of organic carbon. This process, especially the Calvin-Bassham-Benson (CBB) pathway of CO2fixation, provides the bulk of organic carbon found on earth. The enzyme ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase (RubisCO) performs the key and rate-limiting step whereby CO2is reduced and incorporated into a precursor organic metabolite. This is a highly regulated process in diverse organisms, with the expression of genes that comprise the CBB pathway (thecbbgenes), including RubisCO, specifically controlled by the master transcriptional regulator protein CbbR. Many organisms have two or morecbboperons that either are regulated by a single CbbR or employ a specific CbbR for eachcbboperon. CbbR family members are versatile and accommodate and bind many different effector metabolites that influence CbbR's ability to controlcbbtranscription. Moreover, two members of the CbbR family are further posttranslationally modified via interactions with other transcriptional regulator proteins from two-component regulatory systems, thus augmenting CbbR-dependent control and optimizing expression of specificcbboperons. In addition to interactions with small effector metabolites and other regulator proteins, CbbR proteins may be selected that are constitutively active and, in some instances, elevate the level ofcbbexpression relative to wild-type CbbR. Optimizing CbbR-dependent control is an important consideration for potentially using microbes to convert CO2to useful bioproducts.
Kinetics and mechanistic analysis of an extremely rapid carbon dioxide fixation reaction