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.
Hamanaka S., K. Ohtsu, K. Kadawaki, M. Nakazono and A. Hirai, 1999. Identification
of cDNA encoding cytochrome c oxidase subunit 5c (COX5c) from rice:
comparison of its expression with nuclear-encoded and mitochondrial-encoded
COX genes. Genes Genet. Sys. 74: 71-75.
Jansch L., V. Kruft, U.K. Schmitz and H.-P. Braun, 1996. New insights
into the composition, molecular mass and stoichiometry of the protein
complexes of plant mitochondria. Plant J. 9: 357-368.
Kadowaki K., N. Kubo, K. Ozawa and A. Hirai, 1996. Targeting presequence
acquisition after mitochondrial gene transfer to the nucleus occurs by
duplication of existing targeting signal. EMBO J. 15: 6652-6661.
Nakagawa T., M. Maeshima, K. Nakamura and T. Asahi, 1990. Molecular cloning
of a cDNA for the smallest nuclear-encoded subunit of sweet potato cytochrome
c oxidase: analysis with the cDNA of the structure and import into
mitochondria of the subunit. Eur. J. Biochem. 191: 557-561.
Ohtsu K., S. Hamanaka, K. Yamazaki, M. Nakazono and A. Hirai, 1999. Characterization
of a cDNA encoding a novel subunit for cytochrome c oxidase (COX6b)
from rice. Breeding Science. 49: 211-215.