Characterization of two novel nodule-enhanced α-type carbonic anhydrases from Lotus japonicus

Two cDNA clones coding for α-type carbonic anhydrases (CA; EC 4.2.1.1) in the nitrogen-fixing nodules of the model legume Lotus japonicus were identified. Functionality of the full-length proteins was confirmed by heterologous expression in Escherichia coli and purification of the encoded polypeptides. The developmental expression pattern of LjCAA1 and LjCAA2 revealed that both genes code for nodule enhanced carbonic anhydrase isoforms, which are induced early during nodule development. The genes were slightly to moderately down-regulated in ineffective nodules formed by mutant Mesorhizobium loti strains, indicating that these genes may also be involved in biochemical and physiological processes not directly linked to nitrogen fixation/assimilation. The spatial expression profiling revealed that both genes were expressed in nodule inner cortical cells, vascular bundles and central tissue. These results are discussed in the context of the possible roles of CA in nodule carbon dioxide (CO2) metabolism.

Research Highlights
► Two full-length cDNAs coding for nodule-enhanced α-type CAs from L. japonicus.
► This is the first report on the kinetic characterization of plant α-type CAs.
► Transcripts of both genes are highly up-regulated during nodule development.
► Both transcripts are localized in the central, cortex and vascular tissues.
► Roles of these enzymes in the context of CO2 metabolism in nodules are discussed.