The conserved and divergent roles of carbonic anhydrases in the filamentous fungi Aspergillus fumigatus and Aspergillus nidulans

Carbon dioxide (CO₂) and its hydration product bicarbonate (HCO₃^-) are essential molecules in various physiological processes of all living organisms. The reversible interconversion between CO₂ and HCO₃^- is in equilibrium. This reaction is slow without catalyst, but can be rapidly facilitated by Zn ²⁺-metalloenzymes named carbonic anhydrases (CAs). To gain an insight into the function of multiple clades of fungal CA, we chose to investigate the filamentous fungi Aspergillus fumigatus and A. nidulans. We identified four and two CAs in A. fumigatus and A. nidulans, respectively, named cafA-D and canA-B. The cafA and cafB genes are constitutively, strongly expressed whereas cafC and cafD genes are weakly expressed but CO₂-inducible. Heterologous expression of the A. fumigatus cafB, and A. nidulans canA and canB genes completely rescued the high CO₂-requiring phenotype of a Saccharomyces cerevisiae Δnce103 mutant. Only the ΔcafA ΔcafB and ΔcanB deletion mutants were unable to grow at 0.033% CO₂, of which growth defects can be restored by high CO₂. Defects in the CAs can affect Aspergilli conidiation. Furthermore, A. fumigatus ΔcafA, ΔcafB, ΔcafC, ΔcafD and ΔcafA ΔcafB mutant strains are fully virulent in a low-dose murine infection.