Distinct controls of DNA replication and of nuclear division in the cell cycles of the chlorococcal alga Scenedesmus quadricauda (original) (raw)
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Journal of Phycology, 1995
replication in the presence oj 5-ftuorodeoxyuridine (25 mg' L -1). In the control culture, growth processes occurred in several steps with a decreasing rate oj accumulation oj RNA and protein amount approximately at each doubled value oj the preceding step. Osciltations in the rate oj growth processes in the control culture were temporalty related to the initiation oj individual reproductive steps. At each doubling, the celt became committed to triggering ABSTRACT The kinetics oJtwo growth parameters (total RNA and total protein accumulation) was Jollowed in synchronized cultures oj the chlorococcal alga Scenedesmus quadricauda (Turp.) Bréb. under conditions oJinhibited DNA
Protoplasma, 1995
DNA containing structures (cellular, chloroplast and mitochondrial nuclei ) were stained with the fluorochrome DAPI. Fluorescence intensity, as a measure of DNA content, was estimated during the mitotic cycle in synchronized populations of the chlorococcal alga, Scenedesmus quadricauda. In cells ytelding eight daughter cells, three consecutive steps in chloroplast DNA increase occurred over Dne mitotic cycle. The first step was peïormed shortly after releasing the daughter cells, the second and third steps occurred consecutively during the first half of the mitotic cycle. Commitment to chloroplast DNA replication was chronologically separated from commitrnent to division of chloroplast nuclei, revealing that these two chloroplast reproductive steps were under different control mechanisms. The replication of chloroplast DNA occurred at a different time to that of cell-nuclear DNA. The coordination of chloroplast reproductive processes and those in the nucleocytoplasmic compartment were govemed by the mutual trophic and metabolic dependency of these compartments rather than by any direct or feedback control controlled by either of them.
European Journal of Phycology, 2002
In the course of the cell cycles of synchronous cultures of the chlorococcal alga Scenedesmus quadricauda, the following were monitored: total protein and RNA accumulation as a measure of growth processes, the timing of the commitment points at which the cells trigger the sequence of reproductive processes (DNA replication, nuclear and cellular division) and the course of the reproductive processes. The synchronous cultures were grown either under various lighting regimes, or in the temporary presence of specific inhibitors of either proteosynthesis (cycloheximide) or DNA replication (5-fluorodeoxyuridine). By adjusting the length of the light period, the cell cycle could be manipulated. Cell cycle pattems could be altered to give different numbers of sequences of reproductive processes. The extent of their mutual overlap could be influenced and the number of daughter cells produced could be altered. Schematic illustrations of various cell cycle pattems and comparisons with those of higher plants and other algal species are presented.
Journal of Phycology, 1994
they also disturb transcription. The on ly appropriate inhibitors are those that do not attack DNA molecules themselves but, rather, prevent DNA synthesis by inhibition of some step in its metabolic pathway. Hydroxyurea (HU) and 5-ftuorodeoxyuridi ne (FdUrd) are the most widely used inhibitors belonging to this group. HU is a potent metal-chelating and radical-scavenging inhibitor of iron-containing ribonucleotide reductases (EC 1.17.4.1) (Timson 1975, Follmann 1.983, Wawra and. It acts at the beginning of the deoxyribonucleotide metabolic pathway, and DNA synthesis is stopped after exhaustion of the intracellular deoxyribonucleotide pool (Worthington et al. 1975). FdUrd inhibits the enzyme thymidylate synthase (EC 2.1.1.45), whi~h catalyzes the reductive methylation of 2'-deoxyuridylate to form thymidylate (Cisneros et al. 1993), affecting the deoxyribonucleotide metabolic pathway just before the last step. Because pathways for biosynthesis of DNA nucleotides in algal cells (Chlamydomonas, Chlorella, Scenedesmus) are similar to those in higher plant and animal cells (Feller et al. 1980, for review see Follmann 1983), it is usually assumed that these inhibitors could act in algal cells in a similar way as in other organisms.
Plant Science, 1990
The patterns of accumulation of chloroplast and cytoplasmic ribosomal RNA (chl-and cyt-rRNA) were followed in synchronous cultures of the green alga Scenedesmus quadricauda (Turp.) Bréb. Heterotrophic growth conditions and/ar specific inhibitors of protein and rRNA synthesis in chloroplast were used as tools for studying the relationships between these two processes. Heterotrophically grown cells showed the same pattern of accumulation of both chl-and cyt-rRNA as in photoautotrophically grown cultures. Similarly the ratio of chl-/cyt-rRNA was maintained at 0.2. This suggests that light had a trophic function in regulating the synthesis of both rRNA species. The presence of rifampicin during cell division in the dark inhibited the accumulation of chl-rRNA in newly released daughter cells. while the accumulation of cyt-rRNA was not affected. Consequently. the ratio of chl-/cyt-rRNA dropped to 0.1, whereas the same value in untreated darkened daughters increased to 0.4. After washing aut the rifampicin or illuminating the culture, the rates of accumulation of both rRNAs varied in both cultures bot the chl-/cyt-rRNA ratios tended to equilibrate at 0.2 which is characteristic for continuously illuminated cells. The presence of chloramphenicol in heterotrophic cultures caused a complete inhibition of protein. rRNA and DNA synthesis in chloroplast bot the cells .were able to complete at least two cell cycles. When cells in the light and dark were treated with chloramphenicol in the presence of glucose, no differences in the cell cycle and growth were found. These results indicate that concomitant growth. protein synthesis and the accumulation of rRNA in chloroplast are not required for the progress of the algal cell cycle. The synthesis of starch in heterotrophically grown cells with chloramphenicol was the same as in the control heterotrophically growri eDItore. It therefore seems to be the only chloroplast function required for maintaining the algal cell cycle.
Plant Science, 1988
Synchronous cultures of the green alga Scenedesmus quadricauda were grown at different mean irradiances (ranging from 20 W/m2 to 150 W/m2). At these irradiances, the algae were exposed to illumination regimes which differed in the ratio between light and dark intervals (from 02.22 to 24.00 hl. The patterns of accumulation of chloroplast and cytoplasmic. ribosomal RNA (chl-and cyt-rRNA) and variations in their ratio were followed under the above mentioned growth conditions. The chl-rRNA accumulated at a high specific rate even in the dark while the synthesis of cyt-rRNA was depressed. Ccnsequently, the ratio of chl-/cyt-rRNA increased during the dark period to the high value of 0.4. After the start of the light period, this ratio decreased gradually to the low value of 0.2. In continuously illuminated cells, the chl-rRNA and cyt-rRNA were accumulated at the same specific rate so that their ratio remained constant (0.2) during entire cell cycle. However, the absolute amount of cyt-rRNA accumulated in light was about 10 times higher than that of chl-rRNA. In an experiment with 5-fluorodeoxyuridine treated cells. the evidence was provided that DNA replications, nuclear divisions and chloroplast nucleoid fissions interferred with the course of neither chl-rRNA nor cyt-rRNA during the cell cycle. Chloroplastkinesis and cytokinesis were the only reproductive events that prevented the accumulation of both chl-and cyt-rRNA.
Protoplasma, 1996
FdUrd (5-fluorodeoxyuridine), a specific inhibitor of thymidylate synthase, was used to study the relationship between reproductive processes in chloroplast and nucleocytoplasmic compartments of the chlorococcal alga Scenedesmus quadricauda. The courses of DNA replication and nuclear division in both the compartments were followed in populations synchronised by the alternation of light and dark periods. DAPI-staining of DNA-containing structures was used for their visualisation and quantification. In contrast with cellular reproductive events, those in chloroplasts were not substantially affected by the presence of FdUrd (25 J!g/ml). It was shown that FdUrd specifically blocked nucDNA replication but not ptDNA replication. Thus, cells which had attained commitment to ptDNA replication, fission of pt-nuclei and chloroplast kinesis triggered and terrninated these processes while the corresponding cellular processes were blocked. The courses of reproductive processes in chloroplasts were also substantially unaffected in cells grown in the presence of FdUrd for the whole cell cycle. This provided evidence that attainment of commitment to and terrnination of the entire sequence of reproductive events, including chloroplast fission, were controlled by different mechanisms than the reproductive processes in the nucleocytoplasmic compartment.