A Chlamydomonas reinhardtii UV-sensitive mutant uvs15 is impaired in a gene involved in several repair pathways

Insertional mutagenesis in Chlamydomonas reinhardtii: An effective strategy for the identification of new genes involved in the DNA damage response

The formation of double-strand breaks in DNA represents a serious stress for all types of organisms and requires a precisely regulated and organized DNA damage response (DDR) to maintain genetic information and genome integrity. Chlamydomonas reinhardtii possesses the characteristics of both plants and animals and is therefore suitable for the identification of novel genes connected to a wide spectrum of metabolic pathways, including DDR. One very effective tool for the detection and subsequent characterization of new mutants in C. reinhardtii is insertional mutagenesis. We isolated several insertion mutants sensitive to DNA-damaging agents that had disrupted or completely deleted genes with putative functions in the DDR. In most of the analysed mutants, we identified various changes at both ends and even inside the inserted cassette. Using recent information from databases, we were also able to supplement the characteristics of the previously described mutant with a pleiotropic phenotype. In addition, we confirmed the effectiveness of hairpin-PCR as a strategy for the identification of insertion flanking sites and as a tool for the detection of changes at the site of insertion, thus enabling a better understanding of insertion events.

Preparation of efficient excision repair competent cell-free extracts from C. reinhardtii cells

Chlamydomonas reinhardtii is a prospective model system for understanding molecular mechanisms associated with DNA repair in plants and algae. To explore this possibility, we have developed an in vitro repair system from C. reinhardtii cell-free extracts that can efficiently repair UVC damage (Thymine-dimers) in the DNA. We observed that excision repair (ER) synthesis based nucleotide incorporation, specifically in UVC damaged supercoiled (SC) DNA, was followed by ligation of nicks. Photoreactivation efficiently competed out the ER in the presence of light. In addition, repair efficiency in cell-free extracts from ER deficient strains was several fold lower than that of wild-type cell extract. Interestingly, the inhibitor profile of repair DNA polymerase involved in C. reinhardtii in vitro ER system was akin to animal rather than plant DNA polymerase. The methodology to prepare repair competent cell-free extracts described in the current study can aid further molecular characterization of ER pathway in C. reinhardtii.

Adaptive response of a new radioresistant strain ofChlamydomonas reinhardtiiand correlation with increased DNA double-strandbreak rejoining

In order to study the relationship between radioresistance and the adaptive response, we aimed to produce a new strain of Chlamydomonas reinhardtii with characteristics of high radioresistance coupled with a protoplast structure typical for the genus, and the cell-wall-less phenotype to facilitate rapid cell lysis in DNA double-strand break (DSB) assays. The adaptive response of the new strain was investigated using clonogenic and DSB assays. Strain H-3 was derived by mating a radioresistant strain (AK-9-9) with the cell-wall-less mutant CW15 strain and selecting for radioresistance by clonogenic assay. The random amplification of polymorphic DNA (RAPD) molecular marker system was used to evaluate genetic polymorphisms between H-3 and other related C. reinhardtii strains. DSB were estimated using constant-field electrophoresis. Of several mutant strains tested, strain H-3 was shown to be most radioresistant on the basis of dose to give a 90% lethality (LD90) rate and dose to give a 99% lethality rate (LD99). In addition to its high radioresistance and thinner cell wall as compared with that of the other parental strain AK-9-9, H-3 also expressed a radiation-induced adaptive response measured by clonal survival when given a priming dose before a test dose. DSB were also rejoined more rapidly in cells exposed to a priming dose 4 h previously. It is concluded from split-dose experiments that the already highly radioresistant strain H-3 is further capable of 'over recovery' or adaptation to radiation exposure. Accelerated DSB rejoining in cells given a priming dose may underlie the cellular adaptive response in this organism.