Identification of a SulP-type bicarbonate transporter in marine cyanobacteria

Abstract

Cyanobacteria possess a highly effective CO2-concentrating mechanism that elevates CO 2 concentrations around the primary carboxylase, Rubisco (ribulose-1,5-bisphosphate carboxylase͞oxy-genase). This CO 2-concentrating mechanism incorporates lightdependent, active uptake systems for CO 2 and HCO 3 ؊ . Through mutant studies in a coastal marine cyanobacterium, Synechococcus sp. strain PCC7002, we identified bicA as a gene that encodes a class of HCO 3 ؊ transporter with relatively low transport affinity, but high flux rate. BicA is widely represented in genomes of oceanic cyanobacteria and belongs to a large family of eukaryotic and prokaryotic transporters presently annotated as sulfate transporters or permeases in many bacteria (SulP family). Further gain-offunction experiments in the freshwater cyanobacterium Synechococcus PCC7942 revealed that bicA expression alone is sufficient to confer a Na ؉ -dependent, HCO 3 ؊ uptake activity. We identified and characterized three cyanobacterial BicA transporters in this manner, including one from the ecologically important oceanic strain, Synechococcus WH8102. This study presents functional data concerning prokaryotic members of the SulP transporter family and represents a previously uncharacterized transport function for the family. The discovery of BicA has significant implications for understanding the important contribution of oceanic strains of cyanobacteria to global CO 2 sequestration processes.SulP transporters ͉ CO2 sequestration ͉ photosynthesis I t is estimated that some 50% of global primary productivity occurs in the oceans, and marine cyanobacteria contribute significantly to this global CO 2 sequestration process (1). For example, in open oceans located between 40°N and 40°S, photosynthetic CO 2 fixation is dominated by marine cyanobacteria of the Synechococcus and Prochlorococcus genera, and together these species perform 30-80% of primary production (2, 3). The largest nutrient uptake flux encountered by marine cyanobacteria is for dissolved inorganic carbon (Ci), yet relatively little is known about the process of Ci accumulation for photosynthesis in the oceanic cyanobacteria, in contrast to our knowledge of freshwater strains.In aquatic systems, Ci exists mainly as two slowly interconvertible forms, CO 2 and HCO 3 Ϫ . In response to unique restrictions on the rate of CO 2 supply in the aquatic environment, cyanobacteria have evolved a very efficient mechanism for capturing CO 2 and HCO 3 Ϫ for photosynthetic fixation into sugars. The Ci-capturing mechanism functions as a CO 2 -concentrating mechanism because it effectively concentrates CO 2 around the main CO 2 -fixing enzyme, ribulose-1,5-bisphosphate carboxylase͞oxygenase (Rubisco). In the best characterized species, mostly freshwater cyanobacterial strains, the CO 2 -concentrating mechanism consists of several active uptake systems for CO 2 and HCO 3 Ϫ , plus a unique microcompartment called the carboxysome that contains the CO 2 -fixing enzyme, Rubisco (4-6).In recent years, the availability of a...

Interactions between CCM and N2 fixation in Trichodesmium

“…and Synechocystis spp. These five C i acquisition systems comprise two inducible high-affinity HCO 3 -transporters (BCT1, Omata et al 1999;SbtA, Shibata et al 2002), a constitutive low-affinity HCO 3 -transporter (BicA, Price et al 2004), a constitutive low-affinity system to convert CO 2 into HCO 3 -(CO 2 uptake facilitator) at the NDH-1 complex located at the thylakoid membrane (NDH-1 4 , Maeda et al 2002;Price et al 2002;Shibata et al 2001), as well as an inducible high-affinity CO 2 uptake facilitator based on a modified NDH-1 complex (NDH-1 3 , Maeda et al 2002;Shibata et al 2001). All of these measures function to increase the HCO 3 -concentration in the cytosol of the cell.…”

“…This transporter was first described in the marine cyanobacterium Synechococcus PCC7002 (Price et al 2004) as being a low-affinity, high-flux transporter for HCO 3 -. Further characterization revealed that BicA is dependent on a sodium (Na ? )…”

“…, the overall process might mitigate the cytosolic alkalinization resulting from HCO 3 -uptake. The gene encoding BicA appears to be constitutively expressed (Price et al 2004), at least in the freshwater cyanobacteria Synechocystis PCC6803. Only recently, also in Trichodesmium the regulation of CCM genes was investigated over a range of CO 2 concentrations, different temperatures and over the photoperiod (Levitan et al 2010c), showing constitutive gene expression for bicA1, ndhF4 (encoding subunits of BicA and NDH-1 4 ) as well as carboxysomal-related genes.…”

Increased sulfate uptake by E. coli overexpressing the SLC26-related SulP protein Rv1739c from Mycobacterium tuberculosis

“…However, the transmembrane domain structure of neither family is currently known. SLC26-related SulP genes are widely distributed among bacterial species (Felce et al 2004;Price et al 2004), in addition to the widespread expression of SLC26 genes among eukaryotic organisms (Mount et al 2004). Vertebrate SLC26 polypeptides transport a wide range of anions, including Cl − , HCO 3 − , iodide, formate, oxalate, and sulfate, and have been implicated in teleost kidney sulfate excretion and osmoregulation (Renfro et al 1999;Nakada et al 2005;Katoh et al 2006).…”

“…Vertebrate SLC26 polypeptides transport a wide range of anions, including Cl − , HCO 3 − , iodide, formate, oxalate, and sulfate, and have been implicated in teleost kidney sulfate excretion and osmoregulation (Renfro et al 1999;Nakada et al 2005;Katoh et al 2006). SulP sulfate transporters of plants and yeast have been characterized functionally, and evidence of SulP-mediated HCO 3 − transport has been demonstrated in Synechococcus species (Price et al 2004), but little other functional data on bacterial SulP proteins is available. Bacterial proteins have been important for catalyzing progress in the structure determination of polytopic membrane proteins of higher organisms.…”