title: Ultraviolet stress delays chromosome replication in light/dark synchronized cells of the marine cyanobacterium Prochlorococcus marinus PCC9511.
abstract: BACKGROUND
  The marine cyanobacterium Prochlorococcus is very abundant in warm, nutrient-poor oceanic areas. The upper mixed layer of oceans is populated by high light-adapted Prochlorococcus ecotypes, which despite their tiny genome (approximately 1.7 Mb) seem to have developed efficient strategies to cope with stressful levels of photosynthetically active and ultraviolet (UV) radiation. At a molecular level, little is known yet about how such minimalist microorganisms manage to sustain high growth rates and avoid potentially detrimental, UV-induced mutations to their DNA. To address this question, we studied the cell cycle dynamics of P. marinus PCC9511 cells grown under high fluxes of visible light in the presence or absence of UV radiation. Near natural light-dark cycles of both light sources were obtained using a custom-designed illumination system (cyclostat). Expression patterns of key DNA synthesis and repair, cell division, and clock genes were analyzed in order to decipher molecular mechanisms of adaptation to UV radiation.
  
  
  RESULTS
  The cell cycle of P. marinus PCC9511 was strongly synchronized by the day-night cycle. The most conspicuous response of cells to UV radiation was a delay in chromosome replication, with a peak of DNA synthesis shifted about 2 h into the dark period. This delay was seemingly linked to a strong downregulation of genes governing DNA replication (dnaA) and cell division (ftsZ, sepF), whereas most genes involved in DNA repair (such as recA, phrA, uvrA, ruvC, umuC) were already activated under high visible light and their expression levels were only slightly affected by additional UV exposure.
  
  
  CONCLUSIONS
  Prochlorococcus cells modified the timing of the S phase in response to UV exposure, therefore reducing the risk that mutations would occur during this particularly sensitive stage of the cell cycle. We identified several possible explanations for the observed timeshift. Among these, the sharp decrease in transcript levels of the dnaA gene, encoding the DNA replication initiator protein, is sufficient by itself to explain this response, since DNA synthesis starts only when the cellular concentration of DnaA reaches a critical threshold. However, the observed response likely results from a more complex combination of UV-altered biological processes.
authors.affiliation: UPMC-Université Paris 06, Station Biologique, Place Georges Teissier, Roscoff, France.
authors.forename: Christian
authors.initials: C
authors.lastname: Kolowrat
authors.forename: Frédéric
authors.initials: F
authors.lastname: Partensky
authors.forename: Daniella
authors.initials: D
authors.lastname: Mella-Flores
authors.forename: Gildas
authors.initials: G
authors.lastname: Le Corguillé
authors.forename: Christophe
authors.initials: C
authors.lastname: Boutte
authors.forename: Nicolas
authors.initials: N
authors.lastname: Blot
authors.forename: Morgane
authors.initials: M
authors.lastname: Ratin
authors.forename: Martial
authors.initials: M
authors.lastname: Ferréol
authors.forename: Xavier
authors.initials: X
authors.lastname: Lecomte
authors.forename: Priscillia
authors.initials: P
authors.lastname: Gourvil
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Lennon
authors.forename: David M
authors.initials: DM
authors.lastname: Kehoe
authors.forename: Laurence
authors.initials: L
authors.lastname: Garczarek
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002453:Cell Cycle / Q000528:radiation effects; D002876:Chromosomes, Bacterial / Q000235:genetics*; D004261:DNA Replication / Q000528:radiation effects*; D015964:Gene Expression Regulation, Bacterial / Q000528:radiation effects; D017440:Photoperiod; D048929:Prochlorococcus / Q000166:cytology / Q000235:genetics* / Q000378:metabolism / Q000528:radiation effects*; D012623:Seawater / Q000382:microbiology*; D014466:Ultraviolet Rays
chemical_list: D001426:Bacterial Proteins