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

Abstract

Previous studies suggested that certain protein(s) other than carbonic anhydrase might play an important role in the facilitated transport of dissolved inorganic carbon (DIC) from the medium to the site of CO₂ fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase in the unicellular green alga Chlorella regularis adapted to low-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 this facilitated transport of DIC were investigated by pulse-labeling of induced proteins with ³⁵S-sulfate during adaptation of cells grown under high-CO₂ conditions to low CO₂. Analysis by SDS-PAGE revealed that synthesis of two polypeptides, with molecular masses of 98 and 24 kDa, respectively, was induced under low-CO₂ conditions. The 24-kDa polypeptide was induced at pH 5.5 but not at pH 8.0, whereas the 98-kDa polypeptide was induced at both pH 5.5 and pH 8.0. The possible role of these polypeptides in the facilitated transport of DIC in Chlorella regularis is discussed.