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321-327
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Chlorophyll b is a photosynthetic pigment of prochlorophytes and chlorophytes, and it is synthesized by chlorophyllide a oxygenase (CAO) in both organisms. The complete amino acid sequences have been reported for higher plants but not for prochlorophytes. In this study, we determined the complete sequence of a Prochlorothrix CAO (PhCAO) gene by PCR amplification. PhCAO was 164 amino acids shorter than mature Arabidopsis CAO (AtCAO). Based on comparison of amino acid sequences of PhCAO with those of higher plants, we found that CAO in higher plants consists of a catalytic domain and an N-terminal extension. The catalytic domain has binding sites for a Rieske center and a non-heme iron, corresponding to almost the entire sequence of PhCAO. There was high sequence similarity between AtCAO and Oriza sativa CAO (OsCAO). It is thought that photosynthetic possible organisms acquired N-terminal extensions after endosymbiotic events. The function of the N-terminal extension is discussed in this paper.
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