There were other differences in co-expression networks between the two tissues

y genes involved in the regulation of translation, tRNA and aminoacid synthesis had a phase of expression at dawn. Six regulators of translation, including Translation initiation eIF-4G, eIF-3, eIF-3f, the translational repressor Pumilio/PUF3 and related RNA-binding protein and translation ribosome release factor APG3 as well as tRNAs were mainly found in clusters 39/10 7 and 92, suggesting further a possible regulation of protein synthesis at dawn. It is tempting to speculate that the sequential expression of transcription and translation basic machinery, after a gap in global transcription early in the night, may anticipate dawn to ensure a tight regulation of protein synthesis from that time. DNA replication and DNA repair genes are coregulated transcript profiles as DNA replication genes. Like many phytoplanktonic species Ostreococcus cells are exposed to PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19796427 DNA damage due to UV exposure. In our experiment cells were grown at relatively low light intensity lacking UV. It is therefore likely that the transcription of DNA repair genes is directly regulated by the photoperiod and/or by the circadian clock rather than by light intensity. Such a mechanism would be efficient for anticipating photo-damage and repairing DNA upon UV exposure during the day. Several genes involved in chloroplast division such as FtsZ and ARC5 were also detected in S phase clusters. Such a regulation might ensure the coordination of nuclear and chloroplast division, which takes place during nuclear DNA replication. ML-128 custom synthesis Oxidative stress defence and photoprotection genes In Ostreococcus, cell division is synchronized by the photoperiod and several S phase genes were shown to be express at the end of the light period. Ten clusters with phases between time 16.5 and 17.5 were enriched in genes related to DNA replication. The Proliferating Cell Nuclear Antigen, a well known S phase marker, was found in these clusters. Within these 50 S-phase genes, a majority was related to DNA replication, including sister chromatin cohesion proteins, MCM and Origin Recognition complex, DNA polymerase and Ribonuclease H1. DNA replication clusters contained also thymidine kinase and ribonucleotide reductase, which are involved in nucleotide synthesis. Interestingly genes involved in DNA mismatch repair from MLH2/PMS1/Pms2 family, ATR/Tel1 kinase involved in DNA damage signalling or RAD17 had similar We were unable to identify any conventional catalase in the genome of O. tauri suggesting that Ostreococcus uses an alternative mechanism to detoxify reactive oxygen species. Several genes involved in oxidative stress defence and against damaging light environments were found in cluster 85, 3 hours after dawn. Likewise the transcriptional coregulation of UVR3-64 photolyase involved in DNA repair upon UV exposure and the copper/zinc superoxide dismutase known to detoxify reactive oxygen species might be involved in protection against photo-damage. Carotenoids pigments including violaxanthin, zeaxanthin and lutein, which protect cells from photo-oxidative damage, have been described in O. tauri. Interestingly, phytoene desaturase, a precursor enzyme in carotenoid biosynthesis upstream of violaxanthin and zeaxanthin synthesis and zeaxanthin epoxidase, violaxanthin-de-epoxidase and Cytochrome P450 reductase is in cluster7, which also contains enzymes involved in DNA repair such as formamidopyrimidine-DNA glycosylase. Our light intensity condition is more than 10 times lower than intensities O. taur