Limits...
Ultraviolet stress delays chromosome replication in light/dark synchronized cells of the marine cyanobacterium Prochlorococcus marinus PCC9511.

Kolowrat C, Partensky F, Mella-Flores D, Le Corguillé G, Boutte C, Blot N, Ratin M, Ferréol M, Lecomte X, Gourvil P, Lennon JF, Kehoe DM, Garczarek L - BMC Microbiol. (2010)

Bottom Line: 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.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.

View Article: PubMed Central - HTML - PubMed

Affiliation: UPMC-Université Paris 06, Station Biologique, Place Georges Teissier, Roscoff, France.

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.

Show MeSH

Related in: MedlinePlus

Effect of UV exposure on the timing of the cell cycle phases of Prochlorococcus marinus PCC9511 cells grown over a 12 h/12 h light/dark cycle in batch culture. A, distribution of cells in G1 (blue), S (red) and G2 (green) phases for batch cultures of PCC9511 grown under HL. B, same for HL+UV conditions. The experiment was done in duplicates shown by filled and empty symbols. Note that only the UV radiation curve is shown in graph B since the visible light curve is the same as in graph A. White and black bars indicate light and dark periods. The dashed line indicates the irradiance level (right axis). HL, high light; PAR, photosynthetically available radiation; UV, ultraviolet radiation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC2921402&req=5

Figure 1: Effect of UV exposure on the timing of the cell cycle phases of Prochlorococcus marinus PCC9511 cells grown over a 12 h/12 h light/dark cycle in batch culture. A, distribution of cells in G1 (blue), S (red) and G2 (green) phases for batch cultures of PCC9511 grown under HL. B, same for HL+UV conditions. The experiment was done in duplicates shown by filled and empty symbols. Note that only the UV radiation curve is shown in graph B since the visible light curve is the same as in graph A. White and black bars indicate light and dark periods. The dashed line indicates the irradiance level (right axis). HL, high light; PAR, photosynthetically available radiation; UV, ultraviolet radiation.

Mentions: A first series of preliminary experiments using batch cultures of P. marinus PCC9511 was performed in order to examine the effects of UV exposure on cell cycle and growth. Cells were acclimated for several weeks to a modulated 12 h/12 h L/D cycle of photosynthetically available radiation (PAR) reaching about 900 μmol photons m-2 s-1 at virtual noon (HL condition), or with modulated UV radiation added (HL+UV condition), the UV dose at noon reaching 7.6 W m-2 for UV-A and 0.6 W m-2 for UV-B (see additional file 1: Fig. S1). Samples were then taken every hour during three consecutive days and the DNA content of cells was measured by flow cytometry (Fig. 1). In both light conditions, Prochlorococcus population growth conformed to the slow-growth case of Cooper and Helmstetter's prokaryotic cell cycle model [29], with only one DNA replication round per day. Indeed, as described before [6,7], Prochlorococcus DNA distributions always resembled the characteristic bimodal DNA distributions observed for eukaryotes, with a first discrete gap phase (G1), where cells possess one chromosome copy, preceding a well defined chromosome replication phase (S), followed by a second gap phase (G2), where cells have completed DNA replication but have not yet divided, and thus possess two chromosome copies (see additional file 2: Fig. S2). The G1/S/G2 designation will therefore be used in the text hereafter.


Ultraviolet stress delays chromosome replication in light/dark synchronized cells of the marine cyanobacterium Prochlorococcus marinus PCC9511.

Kolowrat C, Partensky F, Mella-Flores D, Le Corguillé G, Boutte C, Blot N, Ratin M, Ferréol M, Lecomte X, Gourvil P, Lennon JF, Kehoe DM, Garczarek L - BMC Microbiol. (2010)

Effect of UV exposure on the timing of the cell cycle phases of Prochlorococcus marinus PCC9511 cells grown over a 12 h/12 h light/dark cycle in batch culture. A, distribution of cells in G1 (blue), S (red) and G2 (green) phases for batch cultures of PCC9511 grown under HL. B, same for HL+UV conditions. The experiment was done in duplicates shown by filled and empty symbols. Note that only the UV radiation curve is shown in graph B since the visible light curve is the same as in graph A. White and black bars indicate light and dark periods. The dashed line indicates the irradiance level (right axis). HL, high light; PAR, photosynthetically available radiation; UV, ultraviolet radiation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2921402&req=5

Figure 1: Effect of UV exposure on the timing of the cell cycle phases of Prochlorococcus marinus PCC9511 cells grown over a 12 h/12 h light/dark cycle in batch culture. A, distribution of cells in G1 (blue), S (red) and G2 (green) phases for batch cultures of PCC9511 grown under HL. B, same for HL+UV conditions. The experiment was done in duplicates shown by filled and empty symbols. Note that only the UV radiation curve is shown in graph B since the visible light curve is the same as in graph A. White and black bars indicate light and dark periods. The dashed line indicates the irradiance level (right axis). HL, high light; PAR, photosynthetically available radiation; UV, ultraviolet radiation.
Mentions: A first series of preliminary experiments using batch cultures of P. marinus PCC9511 was performed in order to examine the effects of UV exposure on cell cycle and growth. Cells were acclimated for several weeks to a modulated 12 h/12 h L/D cycle of photosynthetically available radiation (PAR) reaching about 900 μmol photons m-2 s-1 at virtual noon (HL condition), or with modulated UV radiation added (HL+UV condition), the UV dose at noon reaching 7.6 W m-2 for UV-A and 0.6 W m-2 for UV-B (see additional file 1: Fig. S1). Samples were then taken every hour during three consecutive days and the DNA content of cells was measured by flow cytometry (Fig. 1). In both light conditions, Prochlorococcus population growth conformed to the slow-growth case of Cooper and Helmstetter's prokaryotic cell cycle model [29], with only one DNA replication round per day. Indeed, as described before [6,7], Prochlorococcus DNA distributions always resembled the characteristic bimodal DNA distributions observed for eukaryotes, with a first discrete gap phase (G1), where cells possess one chromosome copy, preceding a well defined chromosome replication phase (S), followed by a second gap phase (G2), where cells have completed DNA replication but have not yet divided, and thus possess two chromosome copies (see additional file 2: Fig. S2). The G1/S/G2 designation will therefore be used in the text hereafter.

Bottom Line: 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.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.

View Article: PubMed Central - HTML - PubMed

Affiliation: UPMC-Université Paris 06, Station Biologique, Place Georges Teissier, Roscoff, France.

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.

Show MeSH
Related in: MedlinePlus