2 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 than carbonic anhydrase might play an important role in the facilitated transport o f dissolved inorganic carbon (DIC) from the medium to the site of CO2 fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase in the uni cellular green alga Chlorella regularis adapted to low-CO2 (ordinary a ir) 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 S-35-sulf ate 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 und er low-CO2 conditions, The 24-kDa polypeptide was induced at pH 5.5 bu t 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 facili tated transport of DIC in Chlorella regularis is discussed.