e. located before the G1-S transition. However, this hypothesis would not account for the previously mentioned small

percentage of the population that was seemingly blocked in S. The occurrence of a “”DNA replication GS-1101 cell line completion checkpoint”" was suggested for UV-C irradiated E. coli cells [56]. Cells in G1 could not start chromosome replication while S cells could not complete replication selleck kinase inhibitor and hence divide; only cells already in G2 at the time of irradiation were able to complete cytokinesis. In our case, however, because of the tight synchronization of the population, virtually no cell was sufficiently advanced in the cell cycle during the pre-dusk period to complete cytokinesis. It is generally thought that checkpoints are controlled by specific protein complexes involved in signaling (photoreceptors) and/or checking [57]. Thus, Prochlorococcus might possess a UV sensor which, when detecting these wavelengths, could launch a cascade of controlling mechanisms ultimately stopping the replication machinery. A UV-B sensor was characterized in the diazotrophic cyanobacterium Chlorogloeopsis sp. PCC6912 and was shown to mediate the induction of mycosporine-like amino acids synthesis [58]. However, no evidence for such a UV sensor is available in Prochlorococcus and, as argued

later in this paper, its presence is rather unlikely. Recently, Cooper [59] proposed that checkpoints may in fact result from purely internal Selleckchem Y-27632 controls. It is possible that PCC9511 cells actually entered the early S phase but that the extensive occurrence of replication fork

stalling due to accumulated DNA lesions and the elevated need for recovery of the replication process by lesion removal and replisome reloading [60] slowed down or even arrested the whole DNA synthesis process for a few hours, therefore explaining the observed delay without any need for a light sensing signal. The fact that UV-acclimated cultures did not show any obvious decrease in their overall growth rate indicates that if stalling of replication forks occurred, efficient DNA repair mechanisms must have allowed those cells blocked in S to restart and complete chromosome replication. UV stress leads to the downregulation of DNA replication and cell division genes To further our understanding of the molecular bases of the observed delay in S phase completion, we analyzed Aspartate the expression of key genes involved in chromosome replication and cell division. As is typically observed in model bacteria [61, 62], the dnaA gene, encoding the master initiator protein of chromosome replication, was induced just before entry of cells into the S phase. Although an increase in dnaA expression occurred at the same time under HL and HL+UV, its level of expression was considerably lower in the latter condition. It is well known in Escherichia coli that initiation of chromosome replication depends on reaching a threshold level of DnaA protein [63].