Relations between the plastid gene dosage and the levels of 16S rRNA and rbcL gene transcripts during amyloplast to chloroplast change in mixotrophic spinach cell suspensions.

Spinach cell suspension cultures maintained in photomixotrophic conditions exhibit plastids which undergo cyclic morphological transformations along a growth cycle. Ultrastructural studies show that the green chloroplasts present at the initial stage differentiate into amyloplasts during the subsequent log phase and then return to chloroplasts in stationary phase. The changes of the levels of plastid DNA (pt DNA) per cell have been determined along the growth cycle, as a percentage of total DNA by hybridization of definite amounts of total DNA to a radioactive probe of cloned pt DNA. The number of pt DNA copies have been estimated to 1125 per cell at the maximum of amyloplast development and to 5940 copies per cell at the maximum of chloroplast differentiation. Hybridizations of defined amounts of total cellular RNA to labelled probes of the 16S rDNA and of the rbcL gene allowed estimations of the variations of the corresponding cellular RNA pools. These variations are well correlated with the changes of the ptDNA cellular levels. These results show that the ptDNA gene dosage plays a central role in the regulation of the plastid transcript levels in this system.