Involvement of the cynABDS Operon and the CO ₂ -Concentrating Mechanism in the Light-Dependent Transport and Metabolism of Cyanate by Cyanobacteria

Abstract

The cyanobacteria Synechococcus elongatus strain PCC7942 and Synechococcus sp. strain UTEX625 decomposed exogenously supplied cyanate (NCO ؊ ) to CO 2 and NH 3 through the action of a cytosolic cyanase which required HCO 3 ؊ as a second substrate. The ability to metabolize NCO ؊ relied on three essential elements: proteins encoded by the cynABDS operon, the biophysical activity of the CO 2 -concentrating mechanism (CCM), and light. (3,28,49). Spontaneous decarboxylation of the carbamate subsequently yields a second CO 2 and NH 3 . Assimilation of cyanate-derived NH 3 and CO 2 then proceeds through conventional metabolic pathways providing a unique source of nitrogen (N) for growth in a variety of bacteria and a source of carbon (C) for autotrophic metabolism (7,15,30,41,50,53).In E. coli, the coexpression of carbonic anhydrase (CA) is vital to maintain ongoing cyanate metabolism (17, 23). Mutants lacking CA activity do not readily catalyze cyanate decomposition, are unable to grow with NCO Ϫ as the sole N source, and are far more susceptible than wild-type cells to the toxic effects of cyanate itself on growth (18,(22)(23)(24). CA involvement is related to the absolute requirement by cyanase for HCO 3 Ϫ (rather than CO 2 ) as a substrate in the reaction. Physiological studies (17,18,22,23) indicate that in the absence of CA, the CO 2 generated from cyanate decomposition diffuses out of cells faster than it can be hydrated nonenzymatically to HCO 3 Ϫ . This leads to a cellular depletion of HCO 3 Ϫ and cessation of cyanate metabolism through substrate deprivation. CA prevents this cellular depletion by trapping CO 2 and catalytically regenerating HCO 3 Ϫ within cells at a rate that is not limiting for cyanase.Carbonic anhydrase, cyanase, and a hydrophobic protein designated as CynX are encoded by cynT, cynS, and cynX (4, 44), respectively, which are arranged in an operon in E. coli, ensuring the coordinated expression of the two enzymes required for cyanate decomposition. Expression of the cynTSX operon is induced by exogenous cyanate and positively regulated by CynR (45), a member of the LysR family of regulatory proteins. The cynR gene is located immediately upstream of the cynTSX operon but is transcribed in the opposite direction.The photoautotrophic cyanobacterium Synechococcus sp. strain UTEX 625 also converts exogenous cyanate to CO 2 and NH 3 as described in the reaction above (30). Inhibitor studies (30) have shown that cyanate-derived NH 3 is rapidly incorporated by this cyanobacterium via the central nitrogen assimilation pathway, and it has recently been suggested that NCO Ϫ can serve as the sole source of N for growth of the globally important marine cyanobacterium Synechococcus sp. strain WH8102 (35,43). CO 2 arising from cyanate decomposition is also rapidly assimilated by Synechococcus sp. strain UTEX 625

The cyanate utilization capacity of marine unicellular Cyanobacteria

“…The cynABD genes required for cyanate acquisition were found only in Synechococcus WH8102 and Prochlorococcus MED4, two oceanic strains associated with oligotrophic surface layers. Genomes of both strains carry the cynA gene (PMM0370 and SYNW2487, respectively), which encodes the periplasmic substrate-binding component of the transporter distinct from other anion ABC-type transporters (Omata et al 2002;Espie et al 2007). The deduced amino sequences predict proteins with a considerably longer N-terminal region than that found in Synechococcus PCC7942 and in orthologs present in the genomes of some photosynthetic bacteria, Rhodopseudomonas palustris, Methylibium petroleiphilum (synonym-Rubrivivax gelatinosus strain PM1), and Roseovarius.…”

“…A cyanate-specific permease was identified, and the knock-out mutants produced were incapable of satisfying their N requirements when cyanate was the sole N source (Sung and Fuchs 1989). Recently, Espie et al (2007) reported an abundance of cyanate transport genes among bacteria. An ABC-type transporter for cyanate was identified in the freshwater cyanobacteria Synechococcus elongatus strain PCC7942 (Harano et al 1997) and Synechococcus sp.…”

Distribution and expression of the cyanate acquisition potential among cyanobacterial populations in oligotrophic marine waters

“…It belongs to the adenosine triphosphate-binding cassette superfamily of permeases (Saier 2000). The periplasmic substrate binding protein CynA is related but phylogenetically distinct from cyanobacterial nitrate and bicarbonate binding proteins (Espie et al 2007). Transcription of cynA is strongly upregulated in both Prochlorococcus sp.…”

Nitrite Transport Activity of the ABC-Type Cyanate Transporter of the Cyanobacterium Synechococcus elongatus

“…Cyanase catalyzes the bicarbonate-dependent decomposition of cyanate (4), and the resulting ammonium and CO 2 can be utilized as nitrogen and carbon sources, respectively, by cyanobacterial cells (31). Because a cynA insertional mutant was as defective in cyanate decomposition as a cynS insertional mutant was, the cynABD genes were identified as the genes for an cyanate transporter (11).…”

“…In this work, we used a nitrogen-responsive promoter-reporter fusion to isolate a mutant defective in NIT activity. It is shown that the three genes cynABD, which encode an ABC-type transporter previously identified as a cyanate (NCO Ϫ ) transporter (11), is responsible for active nitrite transport by NA3.…”