Prochlorococcus and Synechococcus have Evolved Different Adaptive Mechanisms to Cope with Light and UV Stress.

International audienceProchlorococcus and Synechococcus, which numerically dominate vast oceanic areas, are the two most abundant oxygenic phototrophs on Earth. Although they require solar energy for photosynthesis, excess light and associated high UV radiations can induce high levels of oxidative stress that may have deleterious effects on their growth and productivity. Here, we compared the photophysiologies of the model strains Prochlorococcus marinus PCC 9511 and Synechococcus sp. WH7803 grown under a bell-shaped light/dark cycle of high visible light supplemented or not with UV. Prochlorococcus exhibited a higher sensitivity to photoinactivation than Synechococcus under both conditions, as shown by a larger drop of photosystem II (PSII) quantum yield at noon and different diel patterns of the D1 protein pool. In the presence of UV, the PSII repair rate was significantly depressed at noon in Prochlorococcus compared to Synechococcus. Additionally, Prochlorococcus was more sensitive than Synechococcus to oxidative stress, as shown by the different degrees of PSII photoinactivation after addition of hydrogen peroxide. A transcriptional analysis also revealed dramatic discrepancies between the two organisms in the diel expression patterns of several genes involved notably in the biosynthesis and/or repair of photosystems, light-harvesting complexes, CO(2) fixation as well as protection mechanisms against light, UV, and oxidative stress, which likely translate profound differences in their light-controlled regulation. Altogether our results suggest that while Synechococcus has developed efficient ways to cope with light and UV stress, Prochlorococcus cells seemingly survive stressful hours of the day by launching a minimal set of protection mechanisms and by temporarily bringing down several key metabolic processes. This study provides unprecedented insights into understanding the distinct depth distributions and dynamics of these two picocyanobacteria in the field

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

International audienceBACKGROUND: 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

Effet de la forte lumière visible et des radiations ultraviolettes sur le cycle cellulaire et l'incorporation d'acides aminés chez la picocyanobactérie marine Prochlorococcus Marinus PCC9511

La cyanobactérie unicellulaire Prochlorococcus domine la communauté phytoplanctonique dans les gyres océaniques. Le but principal de cette thèse a été d'étudier comment elle s'adapte aux cycles jour/nuit de lumière. Une adaptation clé de Prochlorococcus est sa capacité de photohétérotrophie. Pour savoir si l'incorporation d'acides aminés est un processus continu au cours de la journée, la souche axénique P. marinus PCC9511 a été synchronisée par un cycle J/N modulé et les variations au cours du temps de sa capacité d'incorporation de méthionine et leucine radioactives ont été mesurées. Un rythme nycthéméral pronounce d'incorporation de ces composés a été observé, avec des taux faibles à l'aube et forts au crépuscule, juste avant la division cellulaire. Le coeur de ce travail de thèse a concerné l'effet d'une irradiation par les UV sur la dynamique du cycle cellulaire de PCC9511. L'effet le plus visible des UV était un décalage de la phase de réplication de l'ADN vers la période obscure. Les gènes dont l'expression était la plus perturbée étaient ceux impliquées dans la replication de l'ADN et la division cellulaire. La forte décroissance du niveau d'expression génique de dnaA, qui code pour la protéine d'initiation de la replication serait suffisante en elle-même pour explique le délai observé. Cependant, il est plus probable que ce délai résulte de la combinaison complexe de l'ensemble des processus biologiques altérés par la présence d'UV. Ces résultats apportent une nouvelle lumière sur la vie quotidienne de Prochlorococcus, l'un des organismes clés de la communauté phytoplanctonique de l'océan oligotrophe, et sur sa capacité de survivre dans cet environnement extrême.ROSCOFF-Observ.Océanol. (292393008) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Cystic involvement of the roof of the main biliary convergence in adult patients with congenital bile duct cysts: A difficult surgical challenge

Background. Complete cyst excision of the extrahepatic disease component with biliary reconstruction on proximal healthy bile ducts is considered to be the treatment of choice in patients with congenital bile duct cysts (BDC). Proximal cystic disease that extends to the roof of the main biliary convergence (MBC) might challenge this standard of surgical care. Methods. A retrospective multicenter study was conducted in 4 European surgical centers concerning, their experiment with adult patients suffering from type I and IV BDC according to the Todani classification. Clinical presentation, operative management, and postoperative outcome were compared between patients with or without proximal extrahepatic cystic disease that involved at least the roof Of the MBC (defined as being BDC with MBC involvement subgroup). Results. From an overall series of 49 adult patients suffering from type I or IV BDC according to the Todani classification, 7 patients had BDC with MBC involvement (14%). Patient age, clinical presentation, duration of symptoms, associated major coexistent hepatobiliary and pancreatic diseases, and synchronous cancer were not significantly different in these patients compared with a control group of 42 adult patients with BDC without MBC involvement. Incomplete proximal cyst excision rate was 86% in the cases of BDC with MBC involvement. Early and late postoperative results were similar in BDC with MBC involvement and in the control group of adult patients, but the incidence of subsequent cancer was significantly higher in the BDC with MBC involvement group (29% vs 0%; P < .02). Conclusion. BDC that involves the roof of the MBC is a real surgical challenge to obtain complete proximal cystic disease excision. As suggested in this small study, primary incomplete excision of this particular form of BDC might expose the patient to the risk of subsequent cancer, a feature that must be confirmed in larger series

Primary neuroendocrine tumors of the main pancreatic duct: a rare entity.

International audienceVery few cases of primary neuroendocrine tumors of the main pancreatic duct have been reported. This paucity has hampered an accurate description of the distinctive clinical and pathological features of these tumors and the correct evaluation of the diagnostic and therapeutic problems which they may raise. We report here five additional cases in order to underline the clinical, histological, and immunohistochemical features of this tumor entity. There were three male and two female, aged 43-72 years; in all patients, but one, who presented with epigastric pain, the diagnosis was made after the incidental discovery of a dilatation of the main pancreatic duct. The preoperative diagnosis was ductal adenocarcinoma in one case, IPMN in one case and neoplastic stenosis of unknown etiology in four cases. Surgical resection was performed in all cases. The diagnosis of neuroendocrine tumor was made at histological examination. All lesions were small, ranging from 5 to 15 mm. They had a predominantly intramural growth. The growth pattern was nodular in three cases, circumferential in two; there was no intra-luminal component. All cases were well-differentiated neuroendocrine neoplasms of low histological grade (G1); four cases expressed serotonin. One case was associated with regional lymph node metastases. All cases were cured by surgery alone; no recurrence was observed at the end of the follow-up period. In conclusion, despite their rarity, primary neuroendocrine tumors of the main pancreatic duct deserve recognition and must be considered in the etiological diagnosis of ductal stenosis