A soluble deoxyribonucleic acid in the mosquito Aedes aegypti

Isolation of a single-stranded extrachromosomal DNA from germinating wheat embryos

Newly-synthesized cytoplasmic non-mitochondrial DNA was isolated from wheat embryos which had been germinated in the presence of [14C]-thymidine for a time period not long enough to trigger the first post-dormant round of the nuclear DNA replication. This extrachromosomal DNA fraction consisted of linear single-stranded polydeoxyribonucleotide chains, corresponding in size to approximately 1.1 X 10(6) daltons, and amounted to about 0.5% of the total cellular DNA content. It is suggested that the appearance of the newly-synthesized polydeoxyribonucleotide chains in the cytoplasm may be a physiological signal for the initiation of the nuclear DNA replication in germinating wheat embryo cells.

The biosynthesis of deoxyribonucleic acid during the life-span of the mosquito

Previous results from this laboratory [Kao et al, (1976) Biochem. J. 154, 471-480] demonstrated that a low-molecular-weight DNA species, "sDNA", present in high concentrations during rapid larval growth of the yellow-fever mosquito, Aedes aegypti, may be an intermediate in DNA replication. We have now determined the life-span profiles of synthetic rates and the steady-state contents of sDNA and total DNA by radiometric and biochemical procedures and by using mosquitoes grown and maintained under defined and controlled conditions, including axenic culture and synchronization of stages. 2. Our results showed exponential increases in DNA contents and synthetic rates which paralleled the changes in weight during larval growth. Of special importance was the finding that in rapidly-growing larvae and newly-emerged adult mosquitoes high contents and rates of synthesis of sDNA were correlated with increases in total DNA/organism, which suggests that sDNA content may be an index of DNA replication. In senescent mosquitos a twofold increase in the synthetic rate of bulk DNA ("pDNA") without an increase in sDNA may reflect repair synthesis. 3. The significance of this investigation is that it may be the first quantitative and systematic description of the synthetic rates and contents of both DNA-replication intermediates and total DNA in an axenic, eukaryotic organism rather than a specific cell type. Equally important is that these findings describe DNA changes during growth and also during aging through senescence in an age-characterized organism.

A deoxyribonucleic acid-replication intermediate in the growing mosquito

In previous experiments on growth and aging in the yellow-fever mosquito, Aedes aegypti, a low mol. wt. (500000) DNA species was found in the supernatant fraction after ultracentrifugation of homogenates of rapidly-growing larvae. This DNA species, "sDNA", constituted 30-40% of total DNA in 2-4-day-old larvae, but was less than 5% in older larvae, pupae and adults. We have now isolated and characterized sDNA and initiated experiments to determine its metabolic role. Isolated sDNA has the same physical and chemical characteristics as bulk DNA, "pDNA", and differs only in size. In CsCl isopycnic centrifugation the buoyant densities of sDNA and pDNA were 1.700 and 1.697 g/cm3 respectively. The "melting" temperature of both DNA species was 84 degrees C. Base compositions calculated from these data and other methods were 38.9 mol% of guanine-plus-cytosine for sDNA, and 38.5 for pDNA. Also, the size of newly-synthesized DNA was investigated by pulse-labelling and pulse-chase experiments. In neutral sucrose gradients the labelled DNA component after a 2h pulse had a sedimentation coefficient of about 8S, but after a 4h pulse sedimented in a broad band from 10-19S. In alkaline sucrose gradients a single peak around 7S was observed for pulse times up to 4h. After a 6h pulse and a 1 day "chase", labelled DNA species had sedimentation coefficients ranging from 10-15S in alkaline sucrose, and after a 2-day chase the values were 17-31S, similar to those of pDNA under alkaline conditions. These results suggest that sDNA represents an intermediate form in the replication of DNA in mosquito larvae.

[RNA-Synthesis in telotrophic meroistic ovarioles of dysdercus intermedius DIST. (heteroptera, pyrrhoc.)]

RNA in insect ovaries was investigated by polyacrylamide gel electrophoresis to study site of synthesis, transportation and incorporation in mature eggs. Telotrophic-meroistic ovarioles ofDysdercus intermedius were selected for this work since they could be dissected in distinct portions: the apical one with polyploid nurse cells and the vitellarium with oocytes covered by a follicular epithelium. RNA was labeled by injecting radioactive precursors or incubating isolated ovarioles in vitro.In nurse cells labeled rRNA, tRNA and non-ribosomal RNA were found and evidence was presented for the processing of a 39s rRNA precursor molecule in 2 steps (36s and 32s) into 28s rRNA while the 18s rRNA was directly derived from the 36s molecule. A low concentration (0.5 Μg/ml) of α-Amanitin inhibited synthesis of non-ribosomal RNA thereby revealing a more distinct processing pattern for rRNA, while a high concentration (5 Μg/ml) blocked processing at the 36s molecule and led to its accumulation. Appropriate concentrations of Actinomycin D inhibited rRNA synthesis while distinct peaks of non-ribosomal RNA became apparent.RNA synthesized in the follicular cells was similar to the RNA made in the nurse cells only in the high molecular weight RNA. Some material, tentatively named "nucleotide fraction", with a mobility higher than tRNA was detected and could be localized in yolk granules. It was separable from RNA by chromatography on Sephadex G 100 and had an absorption maximum at 255 run. The "nucleotide fraction" could be rapidly labeled. It seemed to be utilized in RNA synthesis during embryogenesis. Mature eggs contained a relatively high amount of the "nucleotide fraction". If a single injection with precursor was done 4 days before analysing the RNA of mature eggs most of the label was found in the "nucleotide fraction". Eggs collected 5-8 days after injection contained only high molecular labeled RNA. This observation correlated well with the timing of egg maturation. It takes 4 days for a group of oocytes to reach the distal part of the vitellarium where yolk production and maturation go on for another 4 days. It is apparent, therefore, that the follicular epithelium contributes the "nucleotide fraction" to the oocyte, while most of the other RNA, including a stable polydisperse class of RNA, is made in the nurse cells and is transported into the oocyte.