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

Gene expression patterns of L/D-synchronized Prochlorococcus marinus PCC9511 cultures under HL and UV growth conditions, as measured by qPCR. A, dnaA. B, ftsZ. C, sepF. The percentage of cells in the S phase of the cell cycle under HL (solid line) and HL+UV (dashed line) are also shown for comparison. Error bars indicate mean deviation for two biological replicates. For each graph, transcript levels were normalized to the reference time point 6:00 in HL condition. Grey and black bars indicate light and dark periods.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 6: Gene expression patterns of L/D-synchronized Prochlorococcus marinus PCC9511 cultures under HL and UV growth conditions, as measured by qPCR. A, dnaA. B, ftsZ. C, sepF. The percentage of cells in the S phase of the cell cycle under HL (solid line) and HL+UV (dashed line) are also shown for comparison. Error bars indicate mean deviation for two biological replicates. For each graph, transcript levels were normalized to the reference time point 6:00 in HL condition. Grey and black bars indicate light and dark periods.

Mentions: Three genes were selected as representatives of the DNA replication and cell division pathways, dnaA (PMM0565), encoding the DNA replication initiation protein DnaA, ftsZ (PMM1309), encoding the tubulin homolog GTPase protein FtsZ, which forms a ring-shaped septum at midcell during cell division, and sepF (PMM0395), encoding a protein involved in the assembly and stability of the FtsZ ring [32]. The transcript levels of all three genes exhibited strong temporal variations during the diel cycle in both light conditions (Fig. 6). Under HL+UV conditions, although expression levels of both dnaA and ftsZ genes significantly increased at 15:00 compared to the 6:00 time point, the expression level was 3- to 5-fold lower than under HL at 15:00. The sepF gene expression pattern was characterized by a strong peak at the LDT in HL, but like for the other two genes, the diel variations of sepF expression levels were dramatically reduced in UV-irradiated cells. In both light conditions, the sepF expression was maximum during the S phase (Fig. 6C).


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)

Gene expression patterns of L/D-synchronized Prochlorococcus marinus PCC9511 cultures under HL and UV growth conditions, as measured by qPCR. A, dnaA. B, ftsZ. C, sepF. The percentage of cells in the S phase of the cell cycle under HL (solid line) and HL+UV (dashed line) are also shown for comparison. Error bars indicate mean deviation for two biological replicates. For each graph, transcript levels were normalized to the reference time point 6:00 in HL condition. Grey and black bars indicate light and dark periods.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 6: Gene expression patterns of L/D-synchronized Prochlorococcus marinus PCC9511 cultures under HL and UV growth conditions, as measured by qPCR. A, dnaA. B, ftsZ. C, sepF. The percentage of cells in the S phase of the cell cycle under HL (solid line) and HL+UV (dashed line) are also shown for comparison. Error bars indicate mean deviation for two biological replicates. For each graph, transcript levels were normalized to the reference time point 6:00 in HL condition. Grey and black bars indicate light and dark periods.
Mentions: Three genes were selected as representatives of the DNA replication and cell division pathways, dnaA (PMM0565), encoding the DNA replication initiation protein DnaA, ftsZ (PMM1309), encoding the tubulin homolog GTPase protein FtsZ, which forms a ring-shaped septum at midcell during cell division, and sepF (PMM0395), encoding a protein involved in the assembly and stability of the FtsZ ring [32]. The transcript levels of all three genes exhibited strong temporal variations during the diel cycle in both light conditions (Fig. 6). Under HL+UV conditions, although expression levels of both dnaA and ftsZ genes significantly increased at 15:00 compared to the 6:00 time point, the expression level was 3- to 5-fold lower than under HL at 15:00. The sepF gene expression pattern was characterized by a strong peak at the LDT in HL, but like for the other two genes, the diel variations of sepF expression levels were dramatically reduced in UV-irradiated cells. In both light conditions, the sepF expression was maximum during the S phase (Fig. 6C).

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