Molecular characterization of Chlamydomonas reinhardtii telomeres and telomerase mutants

International audienceTelomeres are repeated sequences found at the end of the linear chromosomes of most eukaryotes and are required for chromosome integrity. Expression of the reverse-transcriptase telo-merase allows for extension of telomeric repeats to counteract natural telomere shortening. Although Chlamydomonas rein-hardtii, a photosynthetic unicellular green alga, is widely used as a model organism in photosynthesis and flagella research, and for biotechnological applications, the biology of its telomeres has not been investigated in depth. Here, we show that the C. rein-hardtii (TTTTAGGG) n telomeric repeats are mostly nondegenerate and that the telomeres form a protective structure, with a subset ending with a 39 overhang and another subset presenting a blunt end. Although telomere size and length distributions are stable under various standard growth conditions, they vary substantially between 12 genetically close reference strains. Finally, we identify CrTERT, the gene encoding the catalytic subunit of telomerase and show that telomeres shorten progressively in mutants of this gene. Telomerase mutants eventually enter replicative senescence, demonstrating that telomerase is required for long-term maintenance of telomeres in C. reinhardtii

A Major Role of the RecFOR Pathway in DNA Double-Strand-Break Repair through ESDSA in Deinococcus radiodurans

In Deinococcus radiodurans, the extreme resistance to DNA–shattering treatments such as ionizing radiation or desiccation is correlated with its ability to reconstruct a functional genome from hundreds of chromosomal fragments. The rapid reconstitution of an intact genome is thought to occur through an extended synthesis-dependent strand annealing process (ESDSA) followed by DNA recombination. Here, we investigated the role of key components of the RecF pathway in ESDSA in this organism naturally devoid of RecB and RecC proteins. We demonstrate that inactivation of RecJ exonuclease results in cell lethality, indicating that this protein plays a key role in genome maintenance. Cells devoid of RecF, RecO, or RecR proteins also display greatly impaired growth and an important lethal sectoring as bacteria devoid of RecA protein. Other aspects of the phenotype of recFOR knock-out mutants paralleled that of a ΔrecA mutant: ΔrecFOR mutants are extremely radiosensitive and show a slow assembly of radiation-induced chromosomal fragments, not accompanied by DNA synthesis, and reduced DNA degradation. Cells devoid of RecQ, the major helicase implicated in repair through the RecF pathway in E. coli, are resistant to γ-irradiation and have a wild-type DNA repair capacity as also shown for cells devoid of the RecD helicase; in contrast, ΔuvrD mutants show a markedly decreased radioresistance, an increased latent period in the kinetics of DNA double-strand-break repair, and a slow rate of fragment assembly correlated with a slow rate of DNA synthesis. Combining RecQ or RecD deficiency with UvrD deficiency did not significantly accentuate the phenotype of ΔuvrD mutants. In conclusion, RecFOR proteins are essential for DNA double-strand-break repair through ESDSA whereas RecJ protein is essential for cell viability and UvrD helicase might be involved in the processing of double stranded DNA ends and/or in the DNA synthesis step of ESDSA

The responses of an anaerobic microorganism, Yersinia intermedia MASE-LG-1 to individual and combined simulated Martian stresses

The limits of life of aerobic microorganisms are well understood, but the responses of anaerobic microorganisms to individual and combined extreme stressors are less well known. Motivated by an interest in understanding the survivability of anaerobic microorganisms under Martian conditions, we investigated the responses of a new isolate, Yersinia intermedia MASE-LG-1 to individual and combined stresses associated with the Martian surface. This organism belongs to an adaptable and persistent genus of anaerobic microorganisms found in many environments worldwide. The effects of desiccation, low pressure, ionizing radiation, varying temperature, osmotic pressure, and oxidizing chemical compounds were investigated. The strain showed a high tolerance to desiccation, with a decline of survivability by four orders of magnitude during a storage time of 85 days. Exposure to X-rays resulted in dose-dependent inactivation for exposure up to 600 Gy while applied doses above 750 Gy led to complete inactivation. The effects of the combination of desiccation and irradiation were additive and the survivability was influenced by the order in which they were imposed. Ionizing irradiation and subsequent desiccation was more deleterious than vice versa. By contrast, the presence of perchlorates was not found to significantly affect the survival of the Yersinia strain after ionizing radiation. These data show that the organism has the capacity to survive and grow in physical and chemical stresses, imposed individually or in combination that are associated with Martian environment. Eventually it lost its viability showing that many of the most adaptable anaerobic organisms on Earth would be killed on Mars today

Photoconsommation d'oxygene chez Zea mays L.

National audience18 O2 was used to determine 02-uptake by maize leaves. In this plant, light-dependent 02-uptake was not negligible at the CO2 compensation point. As for C3 plants, 02-uptake increased with 02 concentration; it was strongly limited only by high CO2 concentrations (2 to 4 %). The rate of uptake was also dependent on the stage of plant development. These results suggest that photorespiration is operating in maize leavesA l’aide de l’isotope stable 18 O, il a été possible de mesurer une photoconsommation d’oxygène au point de compensation en CO2 chez le maïs. Comme chez les végétaux de type C3, cette prise d’oxygène augmente en même temps que la concentration en O2 ; par contre, elle n’est fortement diminuée que lorsque la teneur en CO2 est très élevée (2 à 4 p. 100). Elle varie également en fonction du stade de développement du végétal. Ces résultats suggèrent l’existence d’un métabolisme photorespiratoire chez le maï

Complexation des protéines laitières par les extraits de gousses vertes de caroubier (propriétés technologiques des coagulums obtenus)

PARIS-AgroParisTech Centre Paris (751052302) / SudocSudocFranceF

Photoconsommation d'oxygene chez Zea mays L.

National audience18 O2 was used to determine 02-uptake by maize leaves. In this plant, light-dependent 02-uptake was not negligible at the CO2 compensation point. As for C3 plants, 02-uptake increased with 02 concentration; it was strongly limited only by high CO2 concentrations (2 to 4 %). The rate of uptake was also dependent on the stage of plant development. These results suggest that photorespiration is operating in maize leavesA l’aide de l’isotope stable 18 O, il a été possible de mesurer une photoconsommation d’oxygène au point de compensation en CO2 chez le maïs. Comme chez les végétaux de type C3, cette prise d’oxygène augmente en même temps que la concentration en O2 ; par contre, elle n’est fortement diminuée que lorsque la teneur en CO2 est très élevée (2 à 4 p. 100). Elle varie également en fonction du stade de développement du végétal. Ces résultats suggèrent l’existence d’un métabolisme photorespiratoire chez le maï

Incorporation of Oxygen into Glycolate, Glycine, and Serine during Photorespiration in Maize Leaves

Glycolate, glycine, and serine extracted from excised Zea mays L. leaves which had been allowed to photosynthesize in the presence of (18)O(2) were analyzed by gas chromatography-mass spectrometry. In each case, only one of the oxygen atoms of the carboxyl group had become labeled. The maximum enrichment observed in glycine and serine was attained after 5 minutes and 15 minutes of exposure to (18)O(2) at the CO(2) compensation point; the labeling was very high, reaching 70 to 73% of that in the applied O(2). Thus, it appears that all or nearly all of the glycine and serine are synthesized in maize leaves via fixation of O(2). In the presence of CO(2) (380 or 800 microliters per liter), (18)O-labeling was markedly slower. Glycolate enrichment was variable and much lower than that in glycine and serine. It is possible that there are additional pathways of glycolate synthesis which do not result in the incorporation of (18)O from molecular oxygen. An estimation of the metabolic flow of O(2) through the photorespiratory cycle was made. It appeared that less than 75% of the O(2) taken up by maize leaves is involved in this pathway. Therefore, other processes of O(2) metabolism must occur in the light

Photoconsommation d'oxygène chez Zea mays L

International audienc