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International Symposium on Photosynthetic Prokaryotes (2000)
Meeting Report
Global regulator RegB/RegA
by Dan Kramer (Humboldt University)

A number of groups that investigate the facultative phototrophic nonsulfur purple bacteria Rhodobacter capsulatus presented their results concerning the regulation of anaerobic gene expression. In the absence of oxygen, those bacteria can switch from aerobic respiration to anoxygenic photosynthesis. During this change, the two component regulatory system RegB/RegA acts as an activator of photosynthesis and also regulates other anaerobic processes. The RegB/RegA two component system is a global regulator under anaerobic conditions. Within the RegB/RegA system, the RegB protein represents a redox sensor kinase, which appears to directly monitor the respiratory activity of the cytochrome oxidase, while the RegA protein is the response regulator. RegB transmits information regarding the redox state of the enviroment by phosphotransfer to RegA. The autophosphorylation of RegB is a rate-limiting step, and the end result, phosphorylation of RegA, increases the binding of the regulator to DNA by 20-fold.

Carl Bauer (Indiana University) described a variety of RegB/RegA-regulated genes, including those encoding structural proteins of the light harvesting complexes-I and -II and the reaction center. He demonstrated for example that the RegB/RegA system activates the puc promotor (regulating expression of light harvesting-II apoproteins) and the puf promotor (regulating expression of reaction center and light harvesting-I apoproteins). Furthermore, genes encoding proteins involved in carbon fixation, nitrogen fixation and cytochrome biosynthesis belong to the class of RegB/RegA-regulated genes. Biosynthesis of the molybdenum nitrogenase as well as the expression of the uptake hydrogenase are under control of RegB/RegA.

More proteins than the RegB/RegA regulatory circuit itself are involved in the DNA binding process, owing to the weak interactions between the RegA and its target DNA. In her talk, Jutta Gregor (University of Giessen) described proteins identified via the yeast two hybrid system which interact with RegA. The corresponding genes were deleted from the chromosome and the resulting mutants were characterized with regard to oxygen-dependent regulation of photosynthesis.

Although the RegB/RegA two component system seems to be a global regulator, additional redox sensors than RegB exist. They are involved in O2 sensing in order to regulate photosynthetic genes. Gabriele Klug (University of Giessen) presented Rhodobacter strains that are deficient in the sensor kinase RegB but nonetheless exhibit some oxygen-dependent expression of photosynthetic genes. Thioredoxin may be the additional redox sensor, since altered levels of reduced thioredoxin lead to altered oxygen-dependent expression of the puf and puc operons in Rhodobacter sphaeroides. The genome of Rhodobacter capsulatus contain two genes encoding thioredoxin, and their function is currently under investigation.

Other regulators participate with RegB/RegA not only in the regulation of photosynthesis genes but also those of other anaerobic processes. Bauer demonstrated that RegA indirectly activates the synthesis of nitrogenase by activating the expression of nifA2, which encodes the transcriptional activator NifA. Because NtrC is absolutely required for nifA2 expression, RegA acts as a coactivator of nifA2. In the transcriptional regulation of the cbb operon, encoding the form I and form II of the RubisCO, RegA is also involved as an additional transcriptional factor. As Bauer showed, RegA supplements conventional transcriptional activators CbbRI and CbbRII in binding specifically to the cbbI and cbbII promotor-operator regions.