Incorporation of different labelled precursors into chloroplast RNA of Chlorella

Summary Radioactive uridine is incorporated by Chlorella strain 211-8b/p into ribosomal subunits and their rapidly labelled RNA comigrates with chloroplast RNA on polycrylamide gels.Ribosomal particles which can be labelled by short pulses of orotic acid cosediment with the particles labelled by uridine pulses and contain the same RNA species as these when separated either on sucrose gradients or on polycrylamide gels. This incorporation is, like that of uridine, sensitive to rifampin and chloramphenicol, but insensitive to cycloheximide.A comparative study of short-time incorporation of uridine, orotic acid and guanosine into the RNA of Chlorella showed that all three precursors were incorporated mainly into RNA of chloroplastic origin. However, guanosine was also partly incorporated into cytoplasmic rRNA. Nitrogen-deficient cells always incorporated part of all three precursors into cytoplasmic rRNA, but the proportions of these were different among the different precursors.These results are consistent with the hypothesis that the described differences in the incorporation of the above mentioned precursors into RNA of different cellular compartments are largely attributable to effects of pool sizes.     

Changes in DNA composition during cell differentiation in a green alga

In the coenobia-forming green alga, Hydrodictyon reticulatum, changes in DNA composition during growth and differentiation of reproductive cells were followed. A “loss” of DNA per nucleus could be attributed to underreplication of an amplified DNA fraction being part of a (GC)-rich satellite band in CsCl gradients of DNA from young coenobia. This (GC)-rich satellite DNA contains up to 3 % unique DNA sequences intermingled with repetitive DNA. It does not (or nearly not) contain ribosomal DNA. A possible role of this DNA fraction in differentiation is suggested.     

Bevorzugter Einbau markierten Uridins in die Vorläufer von Chloroplasten-Ribosomen in Algenzellen

Summary After short time pulses with 5-[3H]uridine have been given to Chlorella cells, most of the radioactivity of the ribosome fractions is neither in the polysomes nor in the cytoplasmic ribosomes. Peaks with sedimentation of about 50 S and 30 S are found which are comparable in sedimentation to ribosomal subunits of Escherichia coli. During chase treatment with the one-hundred-fold amount of unlabelled uridine, the radioactivity shifts into the 70 S region. The RNA of the rapidly labelled 50 S and 30 S particles is shown to have 23 S, 14 S and 5 S, respectively.In contrast to this, radioactive inorganic phosphate and amino acids are mainly incorporated into the cytoplasmic ribosomes with 80 S and into, their polysomes.The chloroplast-damaged mutant of Chlorella, Nr.125 of Schwarze, shows no uridine incorporation into particles of 50 S and of 30 S, but some very weak labelling of the 80 S cytoplasmic monosomes.Nitrogen deficient Chlorella cells also incorporate uridine mainly into the 50 S and 30 S particles. When chase treatment with unlabelled uridine is performed under recovering conditions, the label shifts into the 70 S particles as well as into the 80 S cytoplasmic ribosomes.The results indicate that in Chlorella, uridine is incorporated into chloroplast ribosome precursors rather than into particles of nuclear origin.     

Synthesis of DNA during the cell cycle of synchronous Anacystis nidulans

Cells of Anacystis nidulans strain 1402-1 incorporate [methyl-³H]thymidine or [8-³H]adenine into DNA; in synchronous cultures (21/2 h full light, 1/2 h weak light, 5 h dark), this incorporation occurs in the dark to different extents according to the labeled precursor offered or to its specific activity. The specific activity of in vivo, uniformly labeled DNA decreases to half the initial value when the cells are grown in the absence of radioactive DNA precursors during the light phase; it does not decrease during the following dark phase. If unlabeled thymidine is given during the dark phase, the specific activity of the DNA starts to decrease at the onset of the next light phase. The time course of the decrease supports the hypothesis that all cells start their DNA replication immediately after illumination and that the first cells have completed if after 1.25 h. The slowest cells then need 3.75 h for completion of DNA replication. It is discussed whether the incorporation during the dark might be due to pool size effects.     

Studies on nucleic acids in plastids of the pigment mutant C-2A{acute} of Scenedesmus obliquus

Pigment mutant C-2A{acute} of Scenedesmus obliquus whose chlorophyllformation and chloroplast development are light dependent, wasstudied for the nucleic acid content of its plastids. The ribosomalRNA of plastids of the achlorophyllous or greened mutant C-2A{acute},did not show any difference from that of the wild type. Incorporationof [5-³H] uridine into mutant cells was partially inhibitedby rifampicin, indicating this part as being plastidial incorporation.Since there were no significant differences in the ribosomalRNA of plastids between the mutant and the wild type of Scenedesmus,the ribosomal system in the plastids of mutant C-2A' seemednot to be affected by the mutation. C_sCl gradient patterns ofScenedesmus mutant and wild-type DNA were almost identical withthose of Chlorella DNA. A peak at a buoyant density of 1.69g/cm³, the same as that of Chlorella chloroplast DNA, couldbe identified in Scenedesmus also as plastid DNA because itdisappeared after prolonged treatment with myxin and hybridizedwith rifampicin-sensitive pulse-labelled RNA. This peak waspresent to nearly the same degree in the mutant and the wildtype, indicating that a larger deficit of plastid DNA did notoccur in the mutant. Whether or not the mutation might be localizedin the plastid genome is discussed. (Received March 19, 1976; )