Nucleotide Sequence of a cDNA Encoding Rice Chloroplastic Carbonic Anhydrase

CA (EC 4.2.1.1) catalyzes the reversible hydration of CO, according to the reaction CO, + H,O HCO, + H+ and represents 1 to 2% of total leaf soluble protein (Okabe et al., 1984). Considerable differences have been reported to exist between the molecular mass of the holoenzyme in monocotyledonous and dicotyledonous species (4245 kD in monocotyledonous plants and between 140 and 250 kD in dicotyledonous plants), and smaller differences exist between the molecular masses of the subunits, 24 to 36 kD (see Sultemeyer et al., 1993, for review). Previous studies using antibodies have also shown that CA from monocotyledonous and dicotyledonous plant species differ in their antigenic cross-reactivity (Okabe et al., 1984; Burnell, 1990). Recently, the primary structure of spinach (Fawcett et al., 19901, pea (Roeske and Ogren, 1990; Majeau and Coleman, 1991), tobacco (Majeau and Coleman, 1992), and, most recently, Arabidopsis thaliana (Raines et al., 1992) have been reported, and the control of expression of maize (Burnell et al., 1990) and A. thaliana (Raines et al., 1992) CA have also been reported. To date only the CA from dicotyledonous plants has been characterized in any detail, and, except for brief reports on the CA of barley and wandering Jew (Tradescantia albiflora Kunth) (Atkins et al., 1972), monocotyledonous CA has largely been ignored. The characterization of rice (Oryza sativa) (a C3 monocotyledonous plant) CA has been conducted to allow comparison of plant CAs at the molecular level. A rice leaf cDNA library was constructed in AZAP and screened using a maize CA cDNA probe (data not shown) and 10 positively hybridizing phage plaques were isolated (Table I). The longest cDNA contained 1148 bp and an open reading frame encoding a preprotein of 273 amino acids. The processing site for the remova1 of the transit peptide was identified by N-terminal sequencing of the purified rice chloroplastic CA and indicated a transit peptide and a mature protein of 63 and 210 amino acid residues, respectively. The amino acid sequence of the mature rice protein exhibits between 59 and 63% identity with pea, tobacco, spinach, and Arabidopsis CA. The transit peptide of rice CA is considerably shorter than the transit peptide of dicotyledonous CAs. It may be significant that the amino acids deleted in the rice CA transit peptide compared with the dicotyledonous CA transit peptides are between amino acid residues 60 and 100 of spinach CA Table 1. Characteristics of CA cDNA from rice