Two Polypeptides Inducible by Low Levels of CO2 in Soluble Protein Fractions from Chlorella regularis Grown at Low or High pH

Previous studies suggested that certain protein(s) other thancarbonic anhydrase might play an important role in the facilitatedtransport of dissolved inorganic carbon (DIC) from the mediumto the site of CO₂ fixation by ribulose-1,5-bisphosphate carboxylase/oxygenasein the unicellular green alga Chlorella regularis adapted tolow-CO₂ (ordinary air) conditions [Shiraiwa et al. (1991) Jpn.J. Phycol. 39: 355; Satoh and Shiraiwa (1992) Research in Photosynthesis,Vol. III, p. 779]. The proteins that might be involved in thisfacilitated transport of DIC were investigated by pulse-labelingof induced proteins with ³⁵S-sulfate during adaptation of cellsgrown under high-CO₂ conditions to low CO₂. Analysis by SDS-PAGErevealed that synthesis of two polypeptides, with molecularmasses of 98 and 24 kDa, respectively, was induced under low-CO₂conditions. The 24-kDa polypeptide was induced at pH 5.5 butnot at pH 8.0, whereas the 98-kDa polypeptide was induced atboth pH 5.5 and pH 8.0. The possible role of these polypeptidesin the facilitated transport of DIC in Chlorella regularis isdiscussed. (Received October 30, 1995; Accepted February 26, 1996)