Characterization and expression analyses of two plastidic enolase genes in rice

To verify the presence of enolase related to the chloroplastic glycolysis in rice, database search was carried out and identified seven putative enolase genes in the rice genome. Among them, OsEno1 and OsEno3 encode long proteins with N-terminal extensions. GFP protein fusions of these N-terminal extensions were both targeted to plastids of onion epidermal cell. Promoter::GUS analysis showed that OsEno3 was highly expressed in young developing leaves, but its expression was drastically decreased during leaf development and greening. On the other hand, the expression of OsEno1 was low and detected in limited portions such as leaf sheath at the tiller base. Recombinant OsEno1 protein showed enolase activity with a pH optimum at pH 8.0, whereas OsEno3 did not exhibit detectable activity. Although it remains obscure if OsEno3 encodes a functional enolase in vivo, our results demonstrate that the entire glycolytic pathway does not operate in rice chloroplasts.