Non-LTR retrotransposons, also known as LINEs, transpose by reverse transcription of an RNA intermediate. Their mechanism of transposition is apparently different from that of retrotransposons and similar to that of proviruses of retroviruses. The I factor is responsible for the I-R system of hybrid dysgenesis in Drosophila melanogaster. Inducer strains contain several functional I factors whereas reactive strains do not. Transposition of I factors can be experimentally induced: they are stable in inducer strains, but transpose at high frequency in the germline of females, known as SF females, produced by crossing reactive females and inducer males. We have constructed an I element, called IviP2, marked with the vermilion gene, the coding sequence of which was interrupted by an intron. Splicing of the intron can only occur in the transcript initiated from the I element promoter. Transposed copies expressing a wild-type vermilion phenotype were recovered in the germline of SF females in which I factors were actively transposing. This indicates that trans-complementation of a defective I element, deficient for the second open reading frame, by functional I factors can occur in the germline of dysgenic females.
Formation of crosslinks in DNA by various mono-and bifunctional furocoumarins plus UVA light in mouse embryo fibroblasts was evaluated by a NaI density gradient centrifugation method. Angelicin and 3-carbethoxypsoralen did not form any crosslinks: however, angelicin was slightly carcinogenic for mouse skin, whereas 3-carbethoxypsoralen was not carcinogenic. Psoralen induced DNA crosslinking, in a dose-dependent fashion and was highly carcinogenic for mouse skin. In contrast to the psoralen-induced photoadducts (120 per lo6 nucleotides) which were left unrepaired, 41% of the 3-carbethoxypsoralen adducts (30 per lo6 nucleotides) were removed during 1 h of dark post-incubation.
The net total uptake of several amino acids at low (0.8-3.1 mumoles/liter) as well as high (800-1200 mumoles/liter) extracellular concentrations, by normal rat liver, a premalignant liver, a solid hepatoma, and the Zajdela ascitic hepatoma cells, has been compared under conditions in which protein synthesis continues. At low amino acid concentrations, the initial (3 min) total uptake of the various amino acids in the Zajdela cells, was 3-10 (average 7) times more, and the intracellular concentration of the labeled amino acids taken up 14-45 (average 31) times more, than in normal liver. At the high amino acid concentrations, the total uptake in the Zajdela cells, at 60-120 min was 2-5 (average 3.5) times more, and the intracellular concentration of the amino acids taken up 8-18 (average 13) times more, than in normal liver; the corresponding values for the premalignant liver and the solid hepatoma were in between those for normal liver and the Zajdela cells. Further, the rate of the total uptake of amino acids, their intracellular concentration, the proportion of the amino acid taken up utilized for protein synthesis, the rate of incorporation of the amino acid taken up into protein, and the cellular growth rate, seemed to be correlated in the four cell/tissue preparations studied. In most cases, the rate of the net uptake fell drastically with time, the uptake virtually stopping after 90-180 min, probably due to lack of serum in the incubation medium.
Zajdela ascitic hepatoma cells are shown to take up pyrimidine bases at much lower rates than obtained in slices from normal rat liver. The rates of uptake of adenine and uridine by the Zajdela cells are, however, as high as in the slices. Like the slices, again, the Zajdela cells take up E. coli RNA and DNA at very low rates but, unlike the slices, thses cells degrade rapidly the RNA taken up. The Zajdela cells resemble parenchymal cell suspensions derived from normal rat liver in regard to the uptake of pyrimidine bases and the ability to degrade heterologous RNA.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.