Abstract
Ultraviolet rays from the sun alter the base pairs of DNA to create mutations. In order to combat these mutations, organisms have evolved genes encoding DNA repair systems. In the non-photosynthetic microorganism, E. coli such DNA repair genes and their response are well- characterised. Photosynthetic organisms such as cyanobacteria experience increased UV as part of their light-harvesting lifestyle. However, their DNA repair response has yet to be characterised. This study used the model cyanobacterium Synechocystis sp. PCC 6803 to investigate the roles of genes that are important in DNA repair following UV damage. Genes in Synechocystis sp. PCC 6803 were selected based on their homology with E. coli genes. These genes are: uvrA, uvrB, uvrC and uvrD which encode proteins that function with the nucleotide excision repair as well as ssb1 and ssb2 which are both predicted to encode proteins that protect ssDNA. Disruption of each of the uvrABCD genes resulted in non-lethal mutations as observed by the growth of each of the single mutant strains under standard conditions. Under stress conditions the mutant strains showed a high sensitivity to UV-B and chemical-induced DNA damage. Results were consistent with the role in E. coli where each of the uvrABCD genes plays a role in DNA repair under similar conditions. Previous studies have indicated that a group of enzymes known as photolyases are the major defense against repair of UV DNA damage. Yet, findings in this current study suggest that the UvrABCD proteins play a key role in the removal of DNA damage. Disrupting both ssb genes resulted in partially segregated strains, this indicates both copies of the gene are essential and required for growth in the absence of stress. The partial mutant strains had increased sensitivity to DNA damage from UV exposure and mitomycin C treatment. Disruption of both copies of ssb resulted in a strain with an unstable phenotype, likely due to the accumulation of mutations. This indicates that both copies of the ssb gene provide a role in the repair of damaged DNA. This differs to E. coli which have a single copy of ssb. The second copy of ssb in cyanobacteria may provide a function unique to cyanobacteria.