Heme oxygenase (HO; EC 1.14.99.3) is an important enzyme that yields biliverdin IXα (BV), carbon monoxide and iron. At least two kinds of HO subfamilies exist in plants. Our previous report revealed that rice (Oryza sativa L.) HO1 (SE5, also named as OsHO1), a major subfamily of HO, is an oxidative stress–response protein, especially upon paraquat exposure. However, whether there exists rice HO2, another subfamily of HO, is still unknown. In the present study, a rice HO2 gene (named as OsHO2) was cloned and characterized. Its genomic sequence consists of four exons and three introns, and encodes a 36.5 kDa protein precursor with a 4.9 kDa N-terminal transit leader peptide. Further results showed that OsHO2 has a conserved HO signature sequence and shares a high amino acid sequence similarity with other identified plant HO2s. The recombinant mature OsHO2 (mOsHO2) protein expressed in Escherichia coli did not exhibit HO activity, which was in contrast with that of mOsHO1. The results of subcellular localization of OsHO2 demonstrated that it was most likely localized in the chloroplasts. Real-time RT-PCR experiment revealed that although OsHO2 mRNA is a much less abundant than that of OsHO1, both of them were expressed in all tested tissues. Importantly, OsHO2 transcripts could be differentially induced by hemin (a substrate of HO), paraquat (in particular), and NaCl treatments. Together, the results suggested that OsHO2 might act downstream in the signal transduction pathways following abiotic stresses in rice.
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