40. Expression of nuclear-encoded genes for cytochrome c oxidase in rice
  K. OHTSU, M. NAKAZONO, N. TSUTSUMI and A. HIRAI

Laboratory of Plant Molecular Genetics, Graduate School of Agricultural and Life Science, The University of Tokyo, Tokyo, 113-8657 Japan

Most proteins that function in the mitochondrial respiratory chain are composed of multi-subunits. Some of t hese subunits are encoded by the nuclear genome and the others are encoded by the mitochondrial genome. Thus, it is thought that these proteins are synthesized by coordinated expression of the nuclear-encoded and mitochondrial-encoded genes, respectively. To investigate the mechanisms of regulation for the coordinated expression of the genes, it is necessary to characterize respiratory genes in both the nuclear and mitochondrial genomes. Of the genes for such proteins, genes in the mitochondrial genome are well characterized; however, the nuclear-encoded respiratory genes are poorly identified in higher plants as of yet.

Cytochrome c oxidase (COX), which is one of the respiratory chain complexes, oxidizes cytochrome c and transfers electrons to molecular oxygen to form molecular water. The COX of higher plants is composed of at least 10 subunits (Jansch et al. 1996). The three largest subunits (COX1, COX2 and COX3) are encoded by the mitochondrial

genome, and the remaining subunits are encoded by the nuclear genome. Mitochondrial-encoded COX genes have been extensively characterized in higher plants including rice, whereas there is less information about the nuclear-encoded genes in higher plants. Only two types of nuclear-encoded COX genes had previously been described: the COX5b genes from rice (Kadowaki et al. 1996) and the COX5c genes from sweet potato (Nakagawa et al. 1990) and rice (Hamanaka et al. 1999). Subsequently, we characterized a cDNA clone encoding a novel subunit of COX, COX6b from rice (Ohtsu et al. 1999).

To investigate the mechanism of coordinated expression of COX from rice, we analyzed the expression of nuclear-encoded COX genes in various rice organs. Total RNA was extracted from leaves and roots of 11-day-old seedlings grown under constant light or darkness, and from leaf blades, leaf sheaths and young panicles of 3-month-old rice plants (grown in a paddy field) and panicles after heading. Using these RNAs, a northern blot analysis was performed (Fig. 1). Probes used were specific for the rice COX5b, COX5c and COX6b genes. The results showed that these genes were expressed in all organs examined with relative differences in the amounts of expression among the organs. In 11-day-old seedlings, the amounts of expression in roots were higher than those in leaves, regardless of whether the seedlings were grown under light or dark conditions. In 3-month-old plants, the highest relative steady-state transcript level was observed in the young panicles followed by leaf blades and leaf sheaths. The signal intensity from panicles after heading was about the same as that from young panicles It is remarkable that all three genes examined here showed almost the same tendencies of expression among the organs. Thus, it is probable that these three genes are co-regulated. Furthermore, it is possible that the expression of all nuclear-encoded COX genes of rice is under the same mechanism of control.

References

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