Nuclear and mitochondrial genes encoding cytochrome c oxidase subunits respond differently to the same metabolic factors

We have identified in the Arabidopsis thaliana Heyhn. nuclear genome a gene encoding COX6a, a homologue of subunit 6a of the mitochondrial cytochrome c oxidase (COX, E.C. 1.9.3.1) from animals and fungi. The expression of this gene and of the previously identified nuclear and mitochondrial genes, respectively, that encode subunit 6b (COX6b) and subunit 2 of the same enzyme was analyzed by northern blot. The analysis indicated that incubation of plants in solutions containing metabolizable sugars produced an increase in transcript levels for the nuclear genes but not for the mitochondrial one. The effect of carbohydrates showed the same time-dependence for COX6a and COX6b. Incubation of plants in solutions with different nitrogen sources also produced changes in expression. The behavior of the nuclear genes was very similar to that previously observed for genes encoding cytochrome c and cytochrome oxidase subunit 5b, suggesting the operation of a common regulatory mechanism for components of the mitochondrial respiratory chain encoded in the nucleus, but not for their counterparts encoded in the mitochondria. Regulation of the expression of mitochondrial genes, if any, may operate at a different level.