The distribution of mitomycin C-induced sister chromatid exchanges in the euchromatin and heterochromatin of the Indian muntjac

The frequency of sister chromatid exchanges (SCEs) induced by mitomycin C (MMC) in Indian Muntjac chromosomes was determined by the fluorescence plus Giemsa (FPG) technique. Using scanning cytophotometry the relative DNA content of each chromosome was measured with and without acid or alkali pretreatments for C-banding. During acid and alkali treatments, euchromatin lost 20 to 30% of its DNA, while heterochromatin lost less than 5%; an intermediate DNA loss was observed for the short arm of the X chromosome. After growth of cells in the presence of MMC during the first cycle and in the presence of bromodeoxyuridine (BrdU) during the first and second cycles of DNA replication, SCEs in the euchromatin were proportional to DNA content. SCEs at the junctions between the neck of the X chromosome and the long and short arms occurred more frequently than expected. A threshold effect for the induction of SCEs was observed in regions resistant to DNA extraction by acid and alkali treatments (i.e., the neck and short arm of the X chromosome). At high concentrations of MMC, the frequency of SCE at each junction appears to plateau at 0.5.

Fine structure of the heterochromatin of the kangaroo rat Dipidomys ordii, and examination of the possible role of actin and myosin in heterochromatin condensation

Biochemical studies have suggested that some actin and myosin may be present in the nucleus. This raises the possibility that heterochromatin condensation might be the result of an actin-myosin rigour type complex. Since ATP dissociates actin and myosin, this possibility could be examined by determining the effect of ATP on heterochromatin condensation. Thin-section electron microscopy showed large amounts of condensed constitutive heterochromatin in the kidney nuclei and somewhat less in the liver nuclei of the kangaroo rat, Dipidomys ordii. Surprisingly, there were some nuclei in the brain which contained no condensed heterochromatin despite the fact that this genome is composed of 50% satellite DNA. Although washing kidney nuclei with solutions of 10 mM Tris-ATP caused marked decondensation of the heterochromatin, when they were washed with Mg-ATP the heterochromatin was more condensed than in the controls. This suggests the decondensation by Tris-ATP is due to its ability to chelate divalent cations and provides no support for condensation of heterochromatin being the result of myosin-actin interaction. Despite being decondensed, the chromatin fibres of heterochromatin were distinct from those of euchromatin. The heterochromatin formed rod-like 19-5 nm fibres, the euchromatin formed random coils of 11-0-nm fibres.

[Differential reactions of heterochromatin and euchromatin to beta-mercaptoethanol]

The responses of interphase and mitotic chromatins to beta-mercaptoethanol were studied. The reaction of X-chromatin of interphase nuclei was expressed in the increase of the percentage of cells with X-chromatin. beta-Merceptoethanol removed a condensing effect induced by spermine on the diffused chromatin of interphase cultured fibroblasts. beta-Mercaptoethanol caused a differential staining of metaphase chromosome when added to the cultured cells 0.5 hours prior to fixation. The treatment of chromosome preparations with different concentrations of beta-merceptoethanol and for different times led to different staining of chromosomes with asur--eosin. Low concentrations and/or short treatment induced G-banding, most typical with moderate concentrations. Higher concentrations and longer treatment prevented G-staining due to the loosening of G-segments.

Reaction of human heterochromatin and euchromatin to spermine

The frequency of occurrence of cell nuclei with X-chromatin and Y-chromatin, as well as with diffuse and coarse chromatin network was studied, using cultured human fibroblasts 47XXY and 47XYY, after treatment of the cell with spermin. This treatment failed to change the percentage of the nuclei with the X- and Y-bodies, but significantly increased the frequency of occurrence of the nuclei, with coarse chromatin. The heterochromatic chromosomal segments retarded in metaphase condensation after the action of 5-bromdesoxyuridine increased the length and number when spermin was added to 5-bromdesoxyuridine. The data obtained were attributed to the different sensitivity of euchromatin to the condensating action of spermin and to inertness of heterochromatin.

[Differential reaction of condensed and diffuse chromatin to polyamines. II. Effect of putrescine on the chromatin of mitotic chromosomes]

The response of euchromatin and heterochromatin to putrescine was studied using chinese hamster mitotic chromosomes with heterochromatic segments delayed in condensation due to BUdR treatment. These heterochromatic segments did not react to the condensing effect of putrescine looking during metaphase still more elongated. Additional decondensed segments occurred in chromosomes. This is interpreted as the condensing effect of putrescine on euchromatic segments which accelerates transition of the cells into metaphase with preserved BUdR-induced chromosomal decondensation.

Mutagenic selectivity of carcinogenic nitroso compounds 2. n-methyl-N-nitrosourethane

The mutagenicity of the carcinogen methylnitrosourethane (MNUr) was examined in Drosophila with a view to the determination of its activity on heterochromatic loci (especially rDNA) relative to those in the euchromatin. Assays were made of the yield of rDNA mutations (bobbed: bb) relative to other X-chromosome recessive lethals and visibles [X(l + v)] in the same male germ cells after treatment with different doses (1-10 mM) and at various stages in spermatogenesis. Dose dependence followed the same pattern for all genic loci and germ cell stages. In all instances, the regression of mutation frequency on injected molar dose was approximately linear, but could better be described by a quadratic dose curve. In contrast, the mutagenicity pattern during spermatogenesis varied according to the target genes. The response of the euchromatic loci reached a peak among the earlier germ cells (probably the spermatocytes), whereas that for the heterochromatic sites (including rDNA) was maximal in mature sperm. Mutagenic selectivity for rDNA with MNUr, as indicated by the percentage bb/X-mutations, was among the highest for the intrinsically reactive carcinogens (alkylating and arylating agents). This correlates with the strong carcinogenicity of MNUr and adds further support to the concept that rDNA mutations might well be a crucial step in cancer initiation.