Regulators of nonsulfur purple phototrophic bacteria and the interactive control of CO₂assimilation, nitrogen fixation, hydrogen metabolism and energy generation

Abstract

For the metabolically diverse nonsulfur purple phototrophic bacteria, maintaining redox homeostasis requires balancing the activities of energy supplying and energy-utilizing pathways, often in the face of drastic changes in environmental conditions. These organisms, members of the class Alphaproteobacteria, primarily use CO2 as an electron sink to achieve redox homeostasis. After noting the consequences of inactivating the capacity for CO2 reduction through the Calvin-Benson-Bassham (CBB) pathway, it was shown that the molecular control of many additional important biological processes catalyzed by nonsulfur purple bacteria is linked to expression of the CBB genes. Several regulator proteins are involved, with the two component Reg/Prr regulatory system playing a major role in maintaining redox poise in these organisms. Reg/Prr was shown to be a global regulator involved in the coordinate control of a number of metabolic processes including CO2 assimilation, nitrogen fixation, hydrogen metabolism and energy-generation pathways. Accumulating evidence suggests that the Reg/Prr system senses the oxidation/reduction state of the cell by monitoring a signal associated with electron transport. The response regulator RegA/PrrA activates or represses gene expression through direct interaction with target gene promoters where it often works in concert with other regulators that can be either global or specific. For the key CO2 reduction pathway, which clearly triggers whether other redox balancing mechanisms are employed, the ability to activate or inactivate the specific regulator CbbR is of paramount importance. From these studies, it is apparent that a detailed understanding of how diverse regulatory elements integrate and control metabolism will eventually be achieved.

Ribulose bisphosphate carboxylase activity and a Calvin cycle gene cluster in Sulfobacillus species

“…The redox state of the cell, and therefore the availability of reduced inorganic substrates in chemoautotrophs, is also related to expression of the CBB cycle, which could act as an electron sink (Shively et al, 1998;Dubbs & Tabita, 2004). A ferrous iron oxidation-minus mutant of S. acidophilus, grown in the presence of iron and yeast extract, did not produce RuBisCO until reversion to iron oxidation after several serial cultures with the same substrates (N. P. Burton & P. R. Norris, unpublished work).…”

Carbon dioxide fixation as a central redox cofactor recycling mechanism in bacteria

“…Although we have excluded the RegSR system as being central to controlling Calvin cycle fluxes in R. palustris in response to redox changes due to H 2 production, there are a number of other regulatory mechanisms that could be involved. For example, high NADH levels can stimulate Calvin cycle phosphoribulokinase activity (19,20), which in turn can stimulate Calvin cycle gene expression through the transcriptional regulator CbbR (15).…”

Photofermentative Hydrogen Production in Outdoor Conditions

“…Purple non-sulfur bacteria (PNSB) are facultative anoxygenic phototrophs belonging to the class of Alphaproteobacteria and include several genera within order of Rhodobacterales, Rhodospiralles and Rhizobiales [3].They are a diverse group of photosynthetic microorganisms that are capable of photobiological hydrogen production under anaerobic, nitrogen limiting conditions. Various species of PNSB were utilized in hydrogen production studies,…”

“…The primary function of CBB pathway is to provide carbon for the cell under photoautotrophic growth on CO2. However, under photoheterotrophic growth on organic carbon, it mainly functions for redox balancing [3,9,10]. The regulatory enzyme of the CO2 fixation is ribulose-l,5-bisphosphate carboxylase/oxygenase (RuBisCO) which catalyzes the conversion of RuBP (ribulose-l,5-bisphosphate) into glyceraldehyde-3-phosphate.…”

“…The second generation is a flat panel airlift PBR made up of two deep-drawn plates glued together while the third generation comprises deep-drawn plates fused together under pressure and heat [29]. Panel PBRs have a short light path and facilitate the measurement of irradiance at the culture surface [28,3,34]. They can be placed vertically or tilted at optimal angles for maximum exposure to direct sunlight [35][36][37] and can be arranged in stacks close to each other, therefore providing a large illuminated area in a small ground area [38].…”