DNA synthesis in mitochondria under model changes of redox conditions --Konstantinov, YM, Subota, IY, Podsosonny, VA, Arziev, AS In spite of extensive studies of mitochondrial gene functioning, the molecular mechanisms that govern the expression of plant mitochondrial genomes in response to changes of metabolic conditions in the cell and the whole organism have still remained poorly understood at present. We have previously reported (MNL 69:63-64, 1995; MNL 70:29-30, 1996; MNL 71:40-41, 1997; MNL 72:33, 1998) on the effect of different redox conditions on mitochondrial genome expression regarding DNA, RNA and protein synthesis in organello. Significant activation of transcription and translation in mitochondria under oxidising conditions, created by addition of potassium ferricyanide, and its profound repression under reducing conditions in the presence of sodium dithionite can indicate the existence of a special mechanism of redox regulation of genetic functions in plant mitochondria.

We showed (MNL 69:63-64, 1995) a significant decrease in the DNA-synthesizing activity in the presence of both oxidising and reducing agents being more prominent in the case of dithionite. Apart from the apparent triggering of the genetic regulatory mechanism by potassium ferricyanide and sodium dithionite, the redox agents used can also modify other functional parameters of mitochondria as well, in particular, the activities of energy transformation and utilization.

The aim of the present work was to examine the mitochondrial DNA synthesis in organello under changes of redox conditions by the addition of carbonyl cyanide chlorophenylhydrazone (CCCP), an uncoupler of oxidative phosphorylation. It is known that in the presence of CCCP the carriers of the mitochondrial respiratory chain are converted to more oxidising states (Muraoka and Slater, BBA 180:221-226, 1969).

The mitochondria were prepared from 3-day-old etiolated seedlings of hybrid VIR42 MV. The isolation of mitochondria and registration of DNA synthesis in organello were the same as described in our note from this issue.

Figure 1 shows that the addition of CCCP caused an activation of DNA synthesis in mitochondria. Such an effect of CCCP presumably results from redox states of the carriers of the respiratory chain. The effects of such redox agents as sodium dithionite and potassium ferricyanide on mitochondrial DNA synthesis in organello were shown to be drastically changed in the presence of this uncoupler. We registered even an activation of DNA synthesis in mitochondria when CCCP was added after sodium dithionite. Therefore the uncoupler of oxidative phosphorylation caused the elimination of the inhibitory effect of such a reducing agent as dithionite on DNA-synthesizing activity. The addition of CCCP also eliminated the inhibition of DNA synthesis in oxidised conditions created by addition of ferricyanide. The results obtained suggest that there is an activation of DNA synthesis in mitochondria under uncoupling conditions. We suggest also that this mechanism of redox control of genetic functions in these organelles may operate efficiently in vivo on the level of mitochondrial DNA replication.

Figure 1.
 
 


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