THE DIFFERENTIAL CAPACITY FOR RNA SYNTHESIS AMONG CHROMOSOMES: A CYTOLOGICAL APPROACH

Temporal coordination of DNA replication with enzyme synthesis in diploid and heteroploid cells

The rate of DNA synthesis in the S phase of growth of synchronized diploid Chinese hamster cells shows two maximums, while in heteroploid hamster cells the DNA replication rate is constant. In diploid cells a reciprocal relationship exists between maximum DNA synthetic rates and maximum lactate dyhydrogenase and thymidine kinase enzyme levels. Enzyme activity in heteroploid cells increases continuously through the cell cycle with no evidence of oscillations. It seems possible that these differences in molecular organization may accompany or precede the transition to heteroploidy.

Effects of [3H]UdR on the cell-cycle progression of L1210 cells

Tritium-labelled uridine [( 3H]UdR) perturbs progression of L1210 cells through the mitotic cycle. The main effect manifests as a slowdown or arrest of a portion of cells in G2 and is already observed 2 hr after addition of 0.5-5.0 microCi/ml of [3H]UdR into cultures. At 2.5-5.0 microCi/ml of [3H]UdR a slowdown of cell progression through S is also apparent. Additionally, there is an increase in the number of cells with DNA values higher than 4C in cultures growing in the presence of [3H]UdR for 8-24 hr. A pulse of [3H]UdR of 2 hr duration labels predominantly (95%) cellular RNA. The first cell-cycle effects (G2 slowdown) are observed when the amount of the incorporated [3H]UdR is such that, on average there are fewer than thirty-six [3H] decays per cell which corresponds to approximately 12-19 rads of radiation. The S-phase slowdown is seen at a dose of incorporated [3H]UdR twice as high as that inducing G2 effects. The specific localization of [3H]UdR in nucleoli, peripheral nucleoplasm and in cytoplasm, as well as differences in the kinetics of the incorporation in relation to phases of the cell cycle are discussed in the light of the differences between the effects of [3H]UdR and [3H]thymidine. Mathematical modelling of the cell-cycle effects of [3H]UdR is provided.

Adaptive response of human lymphocytes to low concentrations of radioactive thymidine

When human lymphocytes were cultured with [3H]thymidine, which acts as a source of low-level chronic radiation, and then exposed to 150 rad of x-rays at 5, 7, 9, or 11 hours before fixation, the yield of chromatid aberrations was less than the sum of the yields of aberrations induced by [3H]thymidine and x-rays separately. Often fewer aberrations were found after exposure to radiation from both sources than were found after exposure to x-rays alone. At the same fixation times, nonradioactive thymidine did not affect the yield of x-ray-induced aberrations. The same phenomenon occurred at earlier fixation times, after exposure to 30 or 40 rad of x-rays and [3H]thymidine. This response is analogous to the adaptive response to alkylating agents whereby prior treatment with small doses for a long period reduces the damage occurring from large doses of similar agents given for a short time.

Can 3H-uridine-induced sister chromatid exchange be used as a measure of transcriptional activity of chromosomes?

In order to test the validity of using 3H-uridine-induced chromosome damage as a measure of the transcriptional activity of chromosomes, experiments were performed to examine the mechanism by which 3H-uridine induces sister chromatid exchanges. DON and B14FAF28 Chinese hamster cells exposed to 3H-uridine showed a dose-dependent increase in SCE frequency, whereas unlabelled uridine produced no increase. 3H-uridine labelling in the presence of increasing concentrations of unlabelled uridine eliminated this increase in a concentration dependent manner. Competition between 3H-uridine and other unlabelled pyrimidines (thymidine and cytidine) was equally as effective; however, unlabelled hypoxanthine had no effect on 3H-uridine-induced SCEs. This suggested that 3H-uridine caused SCEs after interconversion into deoxycytidine followed by incorporation into DNA. Significant incorporation of 3H into DNA was confirmed by direct comparisons of label in the DNA and RNA fractions, and about 90% of this label was found to be in the cytosine fraction of hydrolysates. We therefore conclude that 3H-uridine produces chromosome damage because of the incorporation of a tritiated conversion product into DNA, rather than by exposure of transcriptionally active DNA to 3H-uridine-labelled RNA. We suggest therefore, that the results of earlier experiments, in which 3H-uridine chromosome damage has been used to assay the transcriptional activity of chromosomes, may need to be reinterpreted.

Insect sex chromosomes. VI. A presumptive hyperactivation of the male X chromosome in Acheta domesticus (L.)

The functional status of the X chromosome in Acheta domesticus has been analysed at the whole chromosome level on the basis of (1) 3H-thymidine autoradiography, (2) 5-BrdU/AO fluorescence microscopy (3) in vivo 5-BrdU incorporation and (4) 3H-UdR induced aberrations. The rationale of these techniques in relation to the functional aspect of the X chromosome is that the inactive X chromosome would (1) show asynchrony in DNA synthesis, (2) show differential fluorescence, (3) respond differentially to in vivo 5-BrdU treatment and (4) the active X chromosome would show aberrations when treated with 3H-Uridine. From the results, it appears that the X chromosomes in both male (XO) and female (XX) somatic cells of Acheta are euchromatic (active). Further, the single X in the male is transcriptionally as active as the two X chromosomes in the female. In other words, the single X in the male is hyperactive when compared with the single X in the female. From this it is inferred that the male X chromosome is differentially regulated in order to bring about an equalization of it's gene product(s) to that produced by both Xs in the female. Drosophila melanogaster has a comparable system of dosage compensation. Thus, Acheta is yet another insect showing evidence for an X chromosome regulatory mechanism of dosage compensation. Additionally, it is surmised that sex determination in Acheta is based on an autosomes/X chromosome balance mechanism.

Endogenous DNA polymerase activity in fractionated rat lever chromatin

Chromatin isolated from adult rat liver was fractionated into template active and inactive components by controlled shearing and glycerol gradient centrifugation. The fractionated chromatin was assayed for DNA-dependent DNA polymerase (DNA mucleotidyl transferase EC 2.7.7.7) activity with and without exogenous activated DNA serving as template. With endogenous chromatin as template, it was found that 90% of the endogenous chromatin bound DNA polymerase activity was located in the transcriptionally active fraction of chromatin, while the distribution of DNA polymerase assayed with exogenous activated DNA was found to be 65% in the transcriptionally active and 35% in the inactive fractions. However, when DNA polymerase was solubilized from these fractions by salt extraction, enzyme activity was found to be equally distributed, suggesting that the difference in endogenous DNA polymerase activity observed between eu- and heterochromatin is due to the restricted template found in repressed fractions.