Residual restoration of DNA lesions in Deinococcus radiodurans mutants indicate presence of a bypass UV-repair process

Kurzfassung

The aim of this dissertation was to make use of the differing UV-susceptibility and UV-damage repair capability of the investigated D. radiodurans strains and to identify components of the hypothesized bypass UV-induced damage repair system. This aim was achieved by i) combining biochemical, survival assessment by cultivation and ultrastructural experimental techniques, ii) monitoring repair kinetics of bipyrimidine photoproducts induced by 254 nm- UV radiation and iii) examining and comparing the gene expression profiles of the differing UV-susceptible D. radiodurans strains post-0.5 h-UV irradiation recovery. Previous studies have corroborated UV radiation to be the primary biocidal factor. The results obtained in this dissertation provide evidence that UV radiation engenders synergistic effects in combination with other stressors like temperature, humidity and desiccation (vacuum, respectively). Together with its inherent biocidal activity UV radiation potentiates the biological effectiveness of the associated parameters. Whereas low temperature and vacuum exhibited a moderate biological effectiveness. Supported by the results obtained from the simulated harsh environment, in which desiccated D. radiodurans cells survived 7 d Mars-like cycles of temperature and water activity without UV radiation. Chances of survival for the strains are enlarged, if shielded by dust particles or covered by soil, suggesting that residing in microhabitats below the surface is most probable in a harsh UV climate. Not only genetic equipment but also the applied wavelengths influence D. radiodurans' repair effciency, especially polychromatic UV radiation enables the UV-sensitive class to repair the UV-induced lesions and retain their vitality. A possible explanatory approach for this observed increase in repair efficiency suggests that the wavelength range λ = 315 - 400 nm activates a photosensitive gene or gene cluster that is involved in UV-induced damage repair. A post-0.5 h-recovery phase was suffcient for wild-type strain R1 to repair up to 80% of the total induced DNA photoproducts. Nucleotide excision repair (NER) is given priority over homologeous recombination repair (HR), based on the fact that 1R1A (recA) resumed repair of bipyrimidine dimers but UVs78 (uvrA-1 uvsE) was unable to repair any photoproduct type not even after 2 h, the utmost post-irradiation recovery time point analyzed. The proposed UV-repair process flow: NER effciently removes both cyclobutane bipyrimidine dimers (CPDs) and 6 - 4 bipyrimidine adducts. HR assists removal of CPD from UV-irradiated DNA, but plays a minor role in removal of 6 - 4 adducts. Though the NER is not existent in UVs78 (uvrA- 1 uvsE) up to 70% of both bipyrimidine photoproduct types were equally repaired post-UV (> 315 nm)-irradiation. If the repair ability were due to the existent HR-pathway, a minimum repair of at least CPD should be measurable following UV-254 nm- and UV-(> 200 nm)-radiation. Hence, an additional pathway must be available that allows repair of both photoproduct classes. The core genes of this novel bypass pathway identi- fied DR2438;DR2439;DR2441 and DR2444 as well as DR2445;DR2446. Whereas the assertion that DR1200 and DR1891 may act as putative signal factors to initiate this NER- and HR-independent bypass still has to be verified.