[Radiation biology of structurally different Drosophila melanogaster genes. Report I. The vestigial gene: molecular characteristic of "point" mutations].

The screening of PCR-detected DNA alterations in 9 spontaneous and 59 gamma-ray-, neutron - or neutron + gamma-ray-induced Drosophila vestigial (vg) gene/"point" mutations was carried out. The detected patterns of existence or absence of either of 16 overlapping fragments into which vg gene (15.1 kb, 8 exons, 7 introns) was divided enable us to subdivide all mutants into 4 classes: (i) PCR+ (40.7%) without the detected changes; (ii) "single-site" (33.9%) with the loss of a single fragment; (iii) partial detections (15.2%) as a loss of 2-9 adjacent fragments and (iv) "cluster" mutants (10.2%) having 2-3 independent changes of(ii) and/or (iii) classes. All spontaneous mutants except one were found to be classified as (ii) whereas radiation-induced mutants are represented by all 4 classes whose interrelation is determined by the dose and radiation quality. In particular, the efficacy of neutrons was found to be nine times as large as that of gamma-rays under the "cluster" mutant induction. Essentially, the distribution of DNA changes along the gene is uneven. CSGE-assay of PCR+-exon 3 revealed DNA heteroduplexes in 5 out of 17 PCR+-mutants studied, 2 of which had small deletions (5 and 11 b) and 3 others made transitions (A --> G) as shown by the sequencing. Therefore, gamma-rays and neutrons seem to be significant environmental agents increasing the SNP risk for the population through their action on the germ cells. The results obtained are also discussed within the framework of the track structure theory and the notion of quite different chromatin organization in somatic and germ cells.

[Relative genetic effectiveness of fission neutrons in inducing various types of recessive mutation in Drosophila melanogaster]. / OGE neitronov deleniia pri induktsii retsessivnykh mutatsii raznogo tipa u Drosophila melanogaster.

The RCR-analysis of 53 gamma-Ray- and neutron-induced vg recessive mutations of Drosophila melanogaster combined with complementation assay with the vg[nw83b27] deletion mutation is used to detect precisely the RGE values of neutrons (0.85 MeV) under the chromosome and point (at the DNA level) mutation induction. Simultaneously, the induction-kinetics of gamma-ray- and neutron-induced macrodeletion as well as recessive lethal mutations in the X-chromosome were studied. The results obtained have shown that all genetic end-points increase linearly with gamma-ray or neutron dose. Thereby, the efficacy of neutrons is found to be twice (and more) as large as that of gamma-rays under the all macro- and micro-aberration mutation induction (macrodeletions and recessive lethals in the X-chromosome, multilocus deletions and intragenic deletions as well). Unlike that, the RGE of neutrons are more than twice as low as that of gamma-rays under the gene/point mutation induction. This feature of neutrons have been predicted as far back as in the early days of the radiation genetics (N.W. Timofeeff-Ressovsky, K.G. Zimmer, 1938), but experimentally supported at the DNA level just now.

[The "2 + 1" mechanism as the basis for synergism in the neutron-photon irradiation of the spermatozoal genome in Drosophila melanogaster]. / Mekhanizm "2 + 1" kak osnova singergizma pri neitron-fotonnom obluchenii genoma spermiev Drosophila melanogaster.

Cytogenetic analysis of polytene chromosomes of Drosophila melanogaster locus-specific mutants induced by consecutive neutron-photon irradiation has shown that their genome contains multiple intra- and inter-chromosome exchanges, including triradials, evidencing the synergistic action of such combined exposure. The appearance of the triradials may be only possible on the base of an interaction between a double and a single DNA strand breaks. The important significance of such interaction as the general mechanism for production of chromosome aberrations in irradiated cells of higher eucaryotes had been postulated by N. V. Luchnik as early as 10 years ago, but only nowadays it has been confirmed experimentally.

[The radiation cytogenetics of multilocus deletions and the principles of the superchromomeric organization of eukaryotic euchromatin]. / Radiatsionnaia tsitogenetika mul'tilokusnykh deletsii i printsipy nadkhromomernoi organizatsii éukhromatina éukariot.

Fine cytological analysis of 72 gamma ray- and neutron-induced multilocus deletions (MLD) at the b, cn and vg regions of Drosophila melanogaster polytene chromosomes combined with the complementation assay at the b region is used to detect precisely the size and location of the MLD ends relatively to chromomeres and to each other. The basic principles of the MLD induction are found to be the same for three studied genome regions, showing that the folding of the interphase chromatin at the superchromomeric level is non-random and follows the megasolenoid-rosette model which is presented and discussed.

[The effect of intracellular pH on radiation damage to mammalian cells]. / Vliianie vnutrikletochnogo pH na radiatsionnoe porazhenie kletok mlekopitaiushchikh.

In experiments with Chinese hamster cells and Ehrlich ascites tumor cells it has been shown that a decrease in intracellular pH from 7.0 to 6.0, produced by 0.1 M phosphate buffer, ameliorates the radiation injury to cells (DMF = 1.35) with respect to their clonogenic capacity and induction of chromosome aberrations. The rate of repair of DNA single-strand breaks, within the pH range under study, is invariable.

Relationship between sedimentation behaviour of DNA-protein complexes and DNA single- and double-strand breaks after irradiation with gamma-rays, pulsed neutrons and 12C ions.

The sedimentation behaviour of DNA-protein complexes was studied following irradiation of Chinese hamster cells (V79-4) and human lymphocytes over a wide dose range of 137Cs gamma-rays, pulsed neutrons and accelerated 12C ions. We have shown that the decrease of relative sedimentation velocity of the complexes at low doses is related to the occurrence of single-strand breaks in DNA. Rejoining of the breaks during a repair period increases the sedimentation velocity. At higher doses of radiation, double-strand breaks lead to an increase of sedimentation velocity of DNA-protein complexes. A new method can be devised on the basis of these results enabling estimation of the yields of single- and double-strand breaks in DNA.

[Yield of sister chromatid exchanges and survival of V79-4 Chinese hamster cells irradiated with 0.7 MeV neutrons]. / Vykhod sestrinskikh khromatidnykh obmenov i vyzhivaemost' kletok kitaiskogo khomiachka U79-4 pri obluchenii neitronami s énergiei 0.7 MéV.

The dependence of the survival rate and the number of sister chromatid exchanges (SCEs) in Chinese hamster V79-4 cells on the dose of gamma-rays and neutrons with average energy of 0.7 MeV has been investigated. The value of RBE for neutrons is 5.5. The number of SCEs increased with the dose of gamma-radiation while no induction of SCEs could be detected after neutron irradiation.

[Effect of gamma-irradiation and neutrons on DNA-membrane complexes in mammalian cells]. / Vozdeistvie gamma-izlucheniia i neitronov na DNK-membrannye kompleksy kletok mlekopitaiushchikh.

The first results of the radiobiological studies obtained in a biophysical channel of the IBR-2 reactor at the Joint Institute for Nuclear Research are presented in this report. A study was made of the DNA-membrane complex sedimentation after exposure of Chinese hamster cells V79-4 to gamma-rays within a wide range of doses. The assumption, that had earlier been forwarded by the authors, of the role of double-strand DNA breaks in the alteration of a relative rate of the complex sedimentation upon irradiation of cells with doses above 50 Gy was confirmed.

[Effect of the structure of the cell nucleus chromatin on absorption of low-energy beta-radiation emitted from an incorporated source]. / Vliianie struktury khromatina iadra kletki na pogloshchenie beta-izlucheniia nizkikh énergii ot inkorporirovannogo istochnika.

To study the energy absorption in cell nuclei the analysis was made of the level of registration of beta-particles, emitted by the incorporated tritium, using a liquid scintillation counter. It was shown that the treatment, modifying the chromatin structure, changed the number of beta-particles registered. The pattern of the spectral changes indicated that the effect observed was not the result of the quenching processes. The effect was absent when the source of a higher energy (14C) was used. The authors submit the conceptions concerning the mechanisms of the phenomenon observed which are confirmed by the theoretical studies.