This mutation resulted in the constitutive expression of this operon even under non-inductive conditions, suggesting that the

occurrence of high levels of DNA photolyase and nudix hydrolase in the cells prior to UV treatment conferred these cells with LEE011 mouse better resistance to this stress than wild type cells, which needed some time to synthesize those proteins. In order to exclude the possibility that the PCC9511 RAD001 strain used in our experiments possessed the point mutation described by Osburne and co-workers [68], we used the PCR primers defined by authors to amplify this region directly from cells collected from each duplicate culture of the HL and HL+UV experiments. In all cases, the sequences were the same as for the wild type (L. Garzarek and M. Ratin, unpublished data). It is noteworthy that Zinser and co-workers [14], who studied the diel variations of the whole transcriptome of L/D synchronized

MED4 cultures, observed a very different expression pattern for phrA as we did here (Fig. 7A), with an increase at night and a decrease during the day (see [69]). Since they used a moderate light irradiance, reaching only one fourth of our HL conditions at virtual noon (232 vs. 875 μmol photons m-2 s-1 in the present study), it is possible that high PAR conditions are needed to trigger the synthesis of the DNA photolyase. The uvrA gene showed an expression pattern very similar to that of phrA in both conditions. It encodes the DNA damage recognition component of the UvrABC system which in bacteria and archaea is involved in the nucleotide excision repair pathway (NER) [70]. This Stem Cells inhibitor pathway, which has Ribose-5-phosphate isomerase the ability to repair a wide range of structurally unrelated DNA lesions [71], is seemingly fully functional in P. marinus PCC9511, since it possesses conserved homologs of all three subunits of the UvrABC system. In Zinser and coworkers’ study [14], uvrA transcript levels showed a rapid increase at the beginning of the light period, remained at quasi

steady state during the rest of the day, then decreased at night (see [69]). This indicates that the uvrA system is also activated at moderate light, though it might not need to be adjusted as precisely to the ambient light as under HL. Another essential safeguard of genomic integrity in prokaryotes is the DNA mismatch repair (MMR) pathway, which removes base mispairings, unpaired bases, and small insertion or deletion loops in DNA by the concerted action of MutS-L-H repair proteins [72]. The genome of P. marinus MED4 contains one homolog of mutS, which in E. coli encodes the DNA damage recognition component of the MMR system. Transcript levels of mutS were the lowest at dawn, increased continuously during the light period and decreased at the beginning of the S phase, suggesting that expression of this gene could increase together with the accumulation of UV and/or reactive oxygen species-induced mutations to DNA.