Abstract
Thioredoxin (Trx) is one of major mediators of light reducing power in the stroma of a number of photoautotrophs. On the other hand, the occurrences of chloroplastic Trxs are demonstrated but their target and physiological function are not solved in diatoms. Two pyrenoidal β-type carbonic anhydrases (CA) in the diatom Phaeodactylum tricornutum, PtCA1 and PtCA2, are supposed to be crucial factors for CCM in diatoms. Mature regions of PtCA1 and 2 were synthesized in Escherichia coli and purified. PtCAs were treated with DTT and Trxs from Arabidopsis or P. tricornutum and activity was measured. Oxidization of mPtCAs completely abolished the CA activity but DTT treatment resumed the activity. This DTT dependent activation of oxidized mPtCAs were synergistically stimulated by Trxs. mPtCA1 and 2 possessed two Cys residues besides Cys residues that form Zn ligands. Cys/Ser replaced mutation of mPtCA1 and 2 inhibited the inactivation of PtCAs under oxidation. These results indicate that PtCAs are inactivated in the stroma under the dark by forming intramolecular disulfide bond and activation accompanied by cleavage of this bond is governed by chloroplastic Trx.
