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

Abstract

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. Inactivation of cynS , encoding cyanase, and cynA yielded mutants unable to decompose cyanate. Furthermore, loss of CynA, the periplasmic binding protein of a multicomponent ABC-type transporter, resulted in loss of active cyanate transport. Competition experiments revealed that native transport systems for CO 2 , HCO 3 − , NO 3 − , NO 2 − , Cl − , PO 4 2− , and SO 4 2− did not contribute to the cellular flux of NCO − and that CynABD did not contribute to the flux of these nutrients, implicating CynABD as a novel primary active NCO − transporter. In the S. elongatus strain PCC7942 Δ chpX Δ chpY mutant that is defective in the full expression of the CCM, mass spectrometry revealed that the cellular rate of cyanate decomposition depended upon the size of the internal inorganic carbon (C i ) (HCO 3 − + CO 2 ) pool. Unlike wild-type cells, the rate of NCO − decomposition by the Δ chpX Δ chpY mutant was severely depressed at low external C i concentrations, indicating that the CCM was essential in providing HCO 3 − for cyanase under typical growth conditions. Light was required to activate and/or energize the active transport of both NCO − and C i . Putative cynABDS operons were identified in the genomes of diverse Proteobacteria , suggesting that CynABDS-mediated cyanate metabolism is not restricted to cyanobacteria.