Extracellular CahB1 from Sodalinema gerasimenkoae IPPAS B-353 Acts as a Functional Carboxysomal β-Carbonic Anhydrase in Synechocystis sp. PCC6803

Abstract

Cyanobacteria mostly rely on the active uptake of hydrated CO2 (i.e., bicarbonate ions) from the surrounding media to fuel their inorganic carbon assimilation. The dehydration of bicarbonate in close vicinity of RuBisCO is achieved through the activity of carboxysomal carbonic anhydrase (CA) enzymes. Simultaneously, many cyanobacterial genomes encode extracellular α- and β-class CAs (EcaA, EcaB) whose exact physiological role remains largely unknown. To date, the CahB1 enzyme of Sodalinema gerasimenkoae (formerly Microcoleus/Coleofasciculus chthonoplastes) remains the sole described active extracellular β-CA in cyanobacteria, but its molecular features strongly suggest it to be a carboxysomal rather than a secreted protein. Upon expression of CahB1 in Synechocystis sp. PCC6803, we found that its expression complemented the loss of endogenous CcaA. Moreover, CahB1 was found to localize to a carboxysome-harboring and CA-active cell fraction. Our data suggest that CahB1 retains all crucial properties of a cellular carboxysomal CA and that the secretion mechanism and/or the machinations of the Sodalinema gerasimenkoae carboxysome are different from those of Synechocystis.

Adapting from Low to High: An Update to CO2-Concentrating Mechanisms of Cyanobacteria and Microalgae

“…Notably, when expressed heterologously in Synechocystis sp. PCC 6803, CahB1 acts as a functional carboxysomal CA [ 168 ]. Theoretically, the overall data [ 144 , 166 , 168 ] could be explained by the potential variations in the architecture of the CAs’ system in cyanobacteria from different habitats.…”

“…PCC 6803, CahB1 acts as a functional carboxysomal CA [ 168 ]. Theoretically, the overall data [ 144 , 166 , 168 ] could be explained by the potential variations in the architecture of the CAs’ system in cyanobacteria from different habitats. The “non-model” strains, including S. gerasimenkoae , may exhibit different schemes of CA involvement in the photosynthetic carbon metabolism compared to “model” strains.…”

“…At the same time, S. gerasimenkoae belongs to a group of the so-called “relict” cyanobacteria [ 11 , 13 ]. Therefore, there is a possibility that those cells preserved a variant of the ancient proto-secretion system [ 168 ].…”