Characterisation of Two cDNAs Encoding Carbonic Anhydrase in Maize Leaves

Abstract

Two distinct cDNAs encoding carbonic anhydrase (CA) were isolated from a maize &lgr;gt 11 library. One of these cDNAs, CA1, which consists of a 5´-leader sequence, three repeat sequences and a 3´-non-coding region, is predicted to encode an open reading frame for a polypeptide of 71.3 kDa. The other cDNA, CA2, which has a 5´-leader sequence containing a 276 bp insert compared to CA1, two repeat sequences and a 3´-non-coding region, is predicted to encode an open reading frame for a polypeptide of 59.2 kDa. Nucleotide sequence alignment analysis indicates that the two repeat sequences of CA2 are homologous with repeat sequences 1 and 3 of CA1, respectively.

Four protein bands, with apparent molecular masses of 52, 47, 28 and 27 kDa, were evident in western blot analyses of crude extracts of maize leaf tissue prepared in the absence of Triton X-100 and two protein bands, with apparent molecular masses of 27 and 28 kDa, were detected in western blots of crude extracts prepared in the presence of Triton X-100. These results indicate either that both CA1 and CA2 mRNAs are only partially translated, that the CA1 and CA2 proteins are processed, or a combination of both of these alternatives. Two maize leaf CA1 and CA2 mRNAs detected on northern blots are longer than any other plant CA mRNA reported to date. Possible roles for the two CA isozymes in maize leaves are discussed.