Does single-dose cell resistance to the radio-mimetic zeocin correlate with a zeocin-induced adaptive response in Chlamydomonas reinhardtii strains?

This study aimed to test whether a correlation exists between single-dose resistance to zeocin and the ability to develop a zeocin-induced adaptive response (AR) in Chlamydomonas reinhardtii strains. Three genotypes were used: wild type (WT) strain 137C and two strains (H-3 and AK-9-9), which are highly resistant to radiation based on survival studies. Based on a micro-colony assay, the strains could be arranged according to their single-dose resistance to zeocin as follows: AK-9-9 > H-3 > 137C. However, zeocin induced a similar level of DSB in strains AK-9-9, H-3 and 137C. The radio- and zeocin-resistant strains AK-9-9 and H-3 showed higher DSB rejoining capacity than the WT strain 137C, suggesting that DSB rejoining can at least partly account for different cell survival. Both WT and radio-resistant strains develop zeocin-induced AR involving increased DSB rejoining. The radio- and zeocin-resistant strains AK-9-9 and H-3 again showed higher DSB rejoining capacity than the WT strain 137C. The higher resistance of strains H-3 and AK-9-9 did not abrogate their ability to adapt, albeit with a smaller magnitude as compared to the WT strain. The obtained results characterize new radio-resistant C. reinhardtii strains, which enrich the collection of resistant C. reinhardtii strains.

Acceleration of DNA-double strand rejoining during the adaptive response of Chlamydomonas reinhardtii.

Newly developed constant-field low voltage electrophoresis (adapted for algae cells by us) was applied to quantify the induction and repair of nuclear DNA double-strand breaks, by measuring the movement of DNA out of the starting wells into the electrophoresis gel using a UV-gel scan and computer analysis of DNA-ethidium bromide fluorescense (Syngene; Gene tools). A cell-wall-less mutant strain of Chlamydomonas reinhardtii (CW15) was used; the DNA and proteins are easily accessible because of the lack of an outer cell wall. Our results showed that giving a small priming dose (50 Gy) led to a small acceleration of dsb rejoining. When the magnitude of the priming dose was progressively increased, there was a corresponding decrease in the fraction of damage remaining at 4 hours after radiation exposure (to a test dose of 500 Gy). This indicates an upregulated rejoining of dsb following exposure of cells to the priming dose, which may be related to the strong adaptive response in this organism. Protein synthesis inhibitors were found to reduce the rate of rejoining of dsb, and from earlier results are known to inhibit the adaptive response. Thus, the adaptive response is likely to be dependent on increased dsb rejoining and depends on de novo protein synthesis. The nature of these proteins has not yet been established. C. reinhardtii CW15 is an attractive model system in which to study the underlying mechanisms of the adaptive response to ionizing radiation, and its underlying link with dsb rejoining. The results are interesting both from a basic biological point of view, and as a means to further understand the response of tumour cells to radiation therapy since the adaptive response has been postulated to determine the shape of the "shoulder" region of the survival curve of cells at low doses of radiation.

[Genotoxic effect of cadmium chloride in various test-systems]. / Genotoksicheskii éffekt khloristogo kadmiia v razlichnykh test-sistemakh.

Many industrial regions of Bulgaria are contaminated with cadmium. Induction of various genetic damages by four concentrations of cadmium chloride was studied in various test systems. None of the tested concentrations induced gene mutations in Salmonella typhimurium. An increase in frequency of gene mutations, mitochondrial mutations, and intragene recombination was detected in Saccharomyces cerevisiae treated with the highest cadmium chloride concentration. A clastogenic effect and a significant decrease in mitotic index (MI) were induced in radicle meristem cells of Pisum sativum L. by the two highest cadmium chloride concentrations. Cadmium chloride was also shown to increase the frequency of sex-linked recessive lethals (SLRLs) and dominant lethals (DLs) in Drosophila germ cells. The results obtained in different test systems allow cadmium chloride to be considered a weak mutagen inducing various genetic damages.

Repair of radiation induced DNA damages in unicellular green algae.

Two cell repair systems--photoreactivation and repair of single-strand DNA breaks have been studied using unicellular green algae as a test-system. Effects of the genotype and the intensity of pico/second UV-laser irradiation on the degree of the photoreactivation have been investigated. It has been shown that the lower intensity (I = 8.10(6) W/cm2) effects less the inactivation of living cells comparing with I = 30.10(6) W/cm2, regardless of the genotype. The clearly expressed higher potentials of strains LARG-1 and 260 to produce and repair alterations of the cyclobutane-pyrimidine dimers type have been established. An analysis of DNA degradation during gamma rays irradiation and after incubation has been carried out for investigation the relationship between strains radioresistance and repair of single-strand break. It has been shown that high efficiency of the repair system is characteristic of the resistant strain obtained from chronically irradiated population.