Cyanobacteria have evolved a significant environmental adaptation, known as a CO(2)-concentrating-mechanism (CCM), that vastly improves photosynthetic performance and survival under limiting CO(2) concentrations. The CCM functions to transport and accumulate inorganic carbon actively (Ci; HCO(3)(-), and CO(2)) within the cell where the Ci pool is utilized to provide elevated CO(2) concentrations around the primary CO(2)-fixing enzyme, ribulose bisphosphate carboxylase-oxygenase (Rubisco). In cyanobacteria, Rubisco is encapsulated in unique micro-compartments known as carboxysomes. Cyanobacteria can possess up to five distinct transport systems for Ci uptake. Through database analysis of some 33 complete genomic DNA sequences for cyanobacteria it is evident that considerable diversity exists in the composition of transporters employed, although in many species this diversity is yet to be confirmed by comparative phenomics. In addition, two types of carboxysomes are known within the cyanobacteria that have apparently arisen by parallel evolution, and considerable progress has been made towards understanding the proteins responsible for carboxysome assembly and function. Progress has also been made towards identifying the primary signal for the induction of the subset of CCM genes known as CO(2)-responsive genes, and transcriptional regulators CcmR and CmpR have been shown to regulate these genes. Finally, some prospects for introducing cyanobacterial CCM components into higher plants are considered, with the objective of engineering plants that make more efficient use of water and nitrogen.

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...

Cyanobacteria probably exhibit the widest range of diversity in growth habitats of all photosynthetic organisms. They are found in cold and hot, alkaline and acidic, marine, freshwater, saline, terrestrial, and symbiotic environments. In addition to this, they originated on earth at least 2.5 billion years ago and have evolved through periods of dramatic O2 increases, CO2 declines, and temperature changes. One of the key problems they have faced through evolution and in their current environments is the capture of CO2 and its efficient use by Rubisco in photosynthesis. A central response to this challenge has been the development of a CO2 concentrating mechanism (CCM) that can be adapted to various environmental limitations. There are two primary functional elements of this CCM. Firstly, the containment of Rubisco in carboxysome protein microbodies within the cell (the sites of CO2) elevation), and, secondly, the presence of several inorganic carbon (Ci) transporters that deliver HCO3- intracellularly. Cyanobacteria show both species adaptation and acclimation of this mechanism. Between species, there are differences in the suites of Ci transporters in each genome, the nature of the carboxysome structures and the functional roles of carbonic anhydrases. Within a species, different CCM activities can be induced depending on the Ci availability in the environment. This acclimation is largely based on the induction of multiple Ci transporters with different affinities and specificities for either CO2 or HCO3- as substrates. These features are discussed in relation to our current knowledge of the genomic sequences of a diverse array of cyanobacteria and their ecological environments.

The cyanobacterial CO 2-concentrating mechanism (CCM) allows photosynthesis to proceed in CO 2-limited aquatic environments , and its activity is modulated in response to inorganic carbon (Ci) availability. Real-time reverse transcriptase-PCR analysis was used to examine the transcriptional regulation of more than 30 CCM-related genes in Synechococcus sp. strain PCC7942 with an emphasis on genes encoding high-affinity Ci transporters and carboxysome-associated proteins. This approach was also used to test hypotheses about sensing of Ci limitation in cyanobacteria. The transcriptional response of Synechococcus sp. to severe Ci limitation occurs rapidly, being maximal within 30 to 60 min, and three distinct temporal responses were detected: (a) a rapid, transient induction for genes encoding carboxysome-associated proteins (ccmKLMNO, rbcLS, and icfA) and the transcriptional regulator, cmpR; (b) a slow sustained induction of psbAII; and (c) a rapid sustained induction of genes encoding the inducible Ci transporters cmpABCD, sbtA, and ndhF3-D3-chpY. The Ci-responsive transcripts investigated had half-lives of 15 min or less and were equally stable at high and low Ci. Through the use of a range of physiological conditions (light and Ci levels) and inhibitors such as 3-(3,4-dichlorophenyl)-1,1dimethylurea, glycolaldehyde, dithiothreitol, and ethoxyzolamide, we found that no strict correlation exists between expression of genes known to be induced under redox stress, such as psbAII, and the expression of the Ci-responsive CCM genes. We argue that redox stress, such as that which occurs under highlight stress, is unlikely to be a primary signal for sensing of Ci limitation in cyanobacteria. We discuss the data in relation to current theories of CO 2 sensing in cyanobacteria.

Freshwater cyanobacteria are subjected to large seasonal fluctuations in the availability of nutrients, including inorganic carbon (Ci). We are interested in the regulation of the CO 2 -concentrating mechanism (CCM) in the model freshwater cyanobacterium Synechococcus sp. strain PCC7942 in response to Ci limitation; however, the nature of Ci sensing is poorly understood. We monitored the expression of high-affinity Ci-transporter genes and the corresponding induction of a high-affinity CCM in Ci-limited wild-type cells and a number of CCM mutants. These genotypes were subjected to a variety of physiological and pharmacological treatments to assess whether Ci sensing might involve monitoring of fluctuations in the size of the internal Ci pool or, alternatively, the activity of the photorespiratory pathway. These modes of Ci sensing are congruent with previous results. We found that induction of a high-affinity CCM correlates most closely with a depletion of the internal Ci pool, but that full induction of this mechanism also requires some unresolved oxygen-dependent process.