Biochimica e t Biophysica Acta Els e vi e r Publishing Company, Amste rdam-Printe d in The Ne the rland» BBA 93564 Inability of rifampicin to inhibit circadian rhythmicity in 'Acetabularia despite inhibition of RNA synthesis • Th e maint e nanc e of th e circadian rh3^hm in photosjmth e tic capacity of anucl e at e Ace tabularia^-* raises a que stion: Why is the rh)d:hm independent of the nucle us? It has be e n de monstrate d pre viously that actinomycin D progre ssive ly inhibits the rh)^hm in photosynthe sis in whole algae whe n the duration of the tre atme nt e xce e ds one we e k*, but that this inhibitor doe s not affe ct the rhythm in anucle ate algae *-*. The e ffe ct of actinomycin D has be e n inte rpre te d as a re suit of the inhibition of the synthe sis of a long-Uve d nucle ar me sse nge r RNA (mRNA). This hypothesis has be e n confirme d by e xjje rime nts with ribonucle ase *. The life time of the spécifie nuclear mRNA has been shown to be nucleus dépendent'. However, it is conceivable that the opération of rhythmicity requires not only such a longUved mRNA but, in addition, the transcription of chloroplastic DNA at high frequency, possibly every day, as suggested by EHRET AND TRUCCO'. Therefore, experiments have been undertaken in order to study the effects of rifampicin, a spécifie inhibitor of bacterial**, mitochondriaP" and chloroplastic*^'^* RNA polymer ase, on the rhythm of photosynthetic capacity in Acetabularia (whole cells). It is possible that, in the absence of the nucleus, the transcription of certain polycistronic fragments of the chloroplastic DNA could be switched on and take over the control of the chloroplastic activity. Therefore, exjjeriments on the effects of rifampicin on the circadian rhythm of photosynthetic capacity have been p)erformed with anucleate fragments of Acetabularia as well as with whole algae. The algae, Acetabularia mediterranea or Acetabularia crenulata, were grown under the standard conditions which have been described elsewhere in detaiP^. The lighting régime was 12:12 from 9 a.m. to 9 p.m., and the intensity of the light was 1400 lux. Rifampicin was a gift of CIBA Ltd. Basle. Since rifampicin is photolabile, the solutions were renewed every morning. The photosynthetic capacity was measured at 6000 lux with a Warburg apparatus according to a technique previously published". Duplicate samples of 25 algae were used to measure the photosynthetic capacity at 10 a.m. and at 3 p.m., the middle of the light period. The différence is, therefore, the measure of a fraction of the amplitude of the oscillation. The results of the experiments on photosynthesis are reported in Table I. With whole algae, various combinations of length of treatment and rifampicin con centration have been used, ranging from i ^g/ml for 24 h to i /iglml during 72 h and 20^g/ml for 48 h (Expts. i7a). In no case was a consistent différence between Controls and treated Acetabularia found. O nly in Expt. 7a was a différence in the value of the rhythm observed; this différence seems to be due to t...
Chloroplast DNA (cpDNA) distribution in the giant unicellular, uninucleate alga Acetabularia mediterranea was analyzed with the DNA-specific fluorochrome 4'6-diamidino-2-phenylindole (DAPI) at various stages of the cell cycle. The number of chloroplasts exhibiting DNA/DAPI fluorescence changes during the cell's developmental cycle: (1) all chloroplasts in germlings contain DNA; (2) the number of plastids with DNA declines during polar growth of the vegetative cell; (3) it increases again prior to the transition from the vegetative to the generative phase; (4) several nucleoids of low fluorescence intensity are present in the chloroplasts of the gametes. The temporal distribution of the number of chloroplasts with DNA appears to be linked to the different mode of chloroplast division and growth during the various stages of development. The chloroplast cycle in relation to the cell cycle is discussed.
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