GENOTOXICITY OF HEAVY IONS IN RECOMBINANT BACTERIA

The mutagenic potential of the heavy ion component of the galactic cosmic radiation of space environment is of major concern for astronauts in terms of tumor induction as radiation late effects. The recombinant SWITCH test is based on TA1535 Salmonella typhimurium cells transformed with a bi-cistronic vector harbouring (a) the genes for bioluminescence production from Photobacterium leiognathi under the control of a DNA-damage inducible promoter and (b) the gene for green fluorescent protein from the jellyfish Aequorea victoria under the control of a constitutive promoter. Suchlike genetically modified organism report on the presence of genotoxic conditions by dose dependent increase of bioluminescence induction and on the presence of cytotoxic conditions by dose dependent decrease in GFP fluorescence. By this, it is possible to analyse bacterial inactivation and mutation induction by ionizing radiation in parallel in the same cell within short time. Experiments with heavy ions have been performed with the SWITCH test at GANIL with 75 MeV/A carbon, 95 MeV/A argon, and 29 MeV/A lead. The results obtained clearly show that the numbers of hits (particles per cm2) necessary to inactivate the bacteria (cytotoxicity) depend on LET. The higher the ionisation capacity of the accelerated ion, the less hits resulted in the same test effect, e.g. 37 % survival. For genotoxicity induction it can be seen, that for very high LET radiation the number of hits required is much less then for lower LET radiation (e.g. 1.4x106/cm2 hits for lead versus 1.3x107/cm2 hits for carbon). The power of the genotoxic response seems to be inversely related to LET. While carbon radiation results in a maximal induction of 72.6x, argon radiation leads to a 29.4x value and lead results in only a factor of 4.8. From the experiments with 3 different ions cross sections σ were calculated. Inactivation cross sections (σRCP) rise over the whole LET range under investigation. The same is true for genotoxicity cross sections (σRGP). Cross sections for maximal luminescence emission (σRGP for peak response) differ by a factor of about 10 from cross sections for inactivation; cross sections for doubling the response from untreated cells (σRGP for 2x) are about 3 orders of magnitude higher