[Endogenous variations of productivity in Scenedesmus acutus and their relation to the nucleic acid metabolism].

1. In Scenedesmus acutus Tomaselli, endogenous variations in cell progeny production and chlorophyll formation have been found which are very similar to those previously described in Chlorella by Hesse (Z. Pflanzenphysiol. 67, 58-77, 1972). When the dark phase of the light-dark-cycle is prolonged to a certain extent, cell productivity drops to a minimal value during the next normal light-dark-cycle. If the duration of the supplementary dark treatment comes near to 24 h, cell productivity is almost normal during the next cycle. 2. Nucleic acid labeling with radioactive precursors is very similar in Scenedesmus acutus and Chlorella pyrenoidosa. Short time labeling with uridine results in labeled chloroplastic RNA and DNA, the cytoplasmic RNA being almost unlabeled. With guanosine, both chloroplastic and cytoplasmic RNA as well as DNA are labelled. In nucleic acid separation on acrylamide gels special caution must be taken, since endocellular RNases are particularely active in some cell stages of Scenedesmus. Optimal results are obtained with ripe mother cells; during nucleic acid purification, cell homogenates have to be frozen together with the phenol-cresol mixture. 3. Large differences in guanosine incorporation are found after treatment of the cells with supplementary dark time. After the normal 10:12 h light-dark-cycle, and also after 24 h of supplementarry dark time, much more radioactive guanosine is incorporated into chloroplastic RNA than into cytoplasmic RNA. After 12 h of supplementary dark time, however, cytoplasmic RNA is more extensively labeled than chloroplastic RNA. 4. When the specific radioactivity of guanosine is diluted to one half, the incorporation into the rRNA of cytoplasm and chloroplast is strongly reduced. This is due to the filling up of the guanosine pool in the two compartments. In contrast, DNA labeling is hardly influenced by reduced specific radioactivity of the precursor. This may be interpreted as meaning that the radioactive labeling reflects the rate of DNA synthesis rather than the size of the guanosine pool in the nucleus. Differences found in the labeling of DNA after 12 and 24 h of supplementary dark time can than be interpreted as variations in DNA synthesis rate.