Opposite Regulation of the Copy Number and the Expression of Plastid and Mitochondrial Genes by Light and Acetate in the Green Flagellate Chlorogonium.

In the unicellular green alga Chlorogonium elongatum (Chlamydomonadaceae), the formation of both the photosynthetic and the respiratory apparatus is under the control of light and acetate. Autotrophically cultured cells possess a 3-fold higher copy number of the plastid genes rbcL and psbA than cells cultivated in the dark with acetate (heterotrophic cells). Under mixotrophic conditions (light and acetate), both genes are present at an intermediate level. This pattern is repeated at the mRNA level. The amounts of rbcL and psbA mRNAs are approximately 3-fold higher in autotrophic cells than in heterotrophic ones and are intermediate in mixotrophic cells. As expected, the copy number of the nuclear-encoded rbcS gene is constant irrespective of the applied culture conditions. RbcS mRNA, however, is 7-fold more frequent in autotrophic than in heterotrophic cells. Again, mixotrophic cells show an intermediate level. In contrast to genes encoding plastid proteins, the copy number and transcript level of the mitochondrial cob gene are approximately 5-fold higher in heterotrophic cells than in autotrophic ones. As before, mixotrophic cells take an intermediate position. Therefore, light and acetate control the genes involved in the formation of either the photosynthetic or the respiratory apparatus in a coordinated but opposite manner.