Two uncoupling protein genes of rice (Oryza sativa L.): molecular study reveals the defects in the pre-mRNA processing for the heat-generating proteins of the subtropical cereal.

The recent finding of uncoupling proteins (UCPs) in plants has demonstrated that plant mitochondria contain at least two distinct types of potential heat-generating protein, i.e. uncoupling proteins (UCPs) and alternative oxidases. In this study, we isolated and characterized two rice genes encoding UCPs with the aim of improving cold tolerance of rice through the modification of UCP expression. The two rice nuclear genes, OsUCP1 and OsUCP2, appeared to encode functional UCPs, as far as the amino acid sequences of the gene products showed. However, our study revealed that the processing of the pre-mRNAs of both genes was defective. The defects in the pre-mRNA processing resulted in multiple abnormal forms of the transcripts, whose translation products seemed not to retain normal UCP activities. No OsUCP cDNA clones corresponding to the normal transcripts were detected in our study. A northern analysis, on the other hand, revealed that neither of the OsUCP genes exhibited cold-enhanced expression, whereas the gene expression for an alternative oxidase was strikingly enhanced in cold-treated leaves of rice. Our present data thus indicate that rice leaves, placed in cold environments, do not express functional UCPs.