A gene involved in N-acyl homoserine lactone (N-AHSL) degradation was identified by screening a genomic library of Rhodococcus erythropolis strain W2. This gene, named qsdA (for quorum-sensing signal degradation), encodes an N-AHSL lactonase unrelated to the two previously characterized N-AHSL-degrading enzymes, i.e., the lactonase AiiA and the amidohydrolase AiiD. QsdA is related to phosphotriesterases and constitutes the reference of a novel class of N-AHSL degradation enzymes. It confers the ability to inactivate N-AHSLs with an acyl chain ranging from C 6 to C 14 , with or without substitution at carbon 3. Screening of a collection of 15 Rhodococcus strains and strains closely related to this genus clearly highlighted the relationship between the ability to degrade N-AHSLs and the presence of the qsdA gene in Rhodococcus. Bacteria harboring the qsdA gene interfere very efficiently with quorum-sensing-regulated functions, demonstrating that qsdA is a valuable tool for developing quorum-quenching procedures.

505Plant, Cell and Environment (2001) 24, 505-515 intercellular partial pressure of CO 2 ; PPFD, photosynthetic photon flux density; R, dark respiration; R VPDB , 13 C/ 12 C ratio of standard VPDB; R s , 13 C/ 12 C ratio of sample; Rubisco, ribulose 1,5 bisphosphate carboxylase-oxygenase; RWC, leaf relative water content; VPD, vapour pressure deficit; d 13 C, carbon isotopic composition. INTRODUCTIONCarbon isotope discrimination during leaf CO 2 assimilation has been extensively studied and models have been developed (Farquhar, O'Leary & Berry 1982;Evans et al. 1986). The simple version of these models, which does not include the discrimination during respiration, has been validated for many species, suggesting that the discrimination during respiration is negligible and does not significantly modify the net discrimination during on-line measurements compared to the predicted values (for a recent review see Brugnoli & Farquhar 2000). Yet, the carbon isotope signature of plant dry matter integrates not only the discrimination during net CO 2 assimilation in the light (including CO 2 diffusion from the atmosphere to the chloroplasts, carboxylation, photorespiration and day respiration) but also the discrimination that could occur during the night-time respiration. Therefore, any fractionation during the night and/or the use of heavy or light substrates for dark respiration (releasing 13 C-enriched or 13 C-depleted CO 2 compared with leaf material) should change the isotopic signature of the remaining leaf material. Moreover, when non-photosynthesizing organs are taken into account, the release of 13 C-enriched or 13 C-depleted CO 2 will further contribute to changes in whole-plant carbon isotopic signature. Henderson, von Caemmerer & Farquhar (1992) observed in some C 4 species that the discrimination determined on leaf dry matter was significantly greater than that measured on-line. Using a modelling approach, they proposed that at least a part of this difference could be explained by the fractionation during dark respiration, releasing CO 2 enriched in 13 C relative to the plant material. We obtained similar results on Phaseolus vulgaris (unpublished results) and Nicotiana sylvestris (Duranceau, ABSTRACTThe variations of d 13 C in leaf metabolites (lipids, organic acids, starch and soluble sugars), leaf organic matter and CO 2 respired in the dark from leaves of Nicotiana sylvestris and Helianthus annuus were investigated during a progressive drought. Under well-watered conditions, CO 2 respired in the dark was 13 C-enriched compared to sucrose by about 4‰ in N. sylvestris and by about 3‰ and 6‰ in two different sets of experiments in H. annuus plants. In a previous work on cotyledonary leaves of Phaseolus vulgaris, we observed a constant 13 C-enrichment by about 6‰ in respired CO 2 compared to sucrose, suggesting a constant fractionation during dark respiration, whatever the leaf age and relative water content. In contrast, the 13 C-enrichment in respired CO 2 increased in dehydrated N. sylvestris and decrea...

In the yeast Saccharomyces cerevisiae, CTP synthetase [EC 6.3.4.2; UTP:ammonia ligase (ADP-forming)] is the product of the URA7 gene. CTP synthetase was purified 503-fold to apparent homogeneity from cells bearing the URA7 gene on a multicopy plasmid that directed a 10-fold overproduction of the enzyme. The purification procedure included ammonium sulfate fractionation of the cytosolic fraction followed by chromatography with Sephacryl 300 HR, Q-Sepharose, Affi-Gel Blue, and Superose 6. The N-terminal amino acid sequence of purified CTP synthetase was identified and aligned perfectly with the deduced sequence of the URA7 gene. The minimum subunit molecular mass (68 kDa) of purified CTP synthetase was in good agreement with the size (64.7 kDa) of the URA7 gene product. Antibodies were raised against a maltose-binding protein-CTP synthetase fusion protein which immunoprecipitated CTP synthetase from wild-type cells. Immunoblot analysis was used to identify CTP synthetase in wild-type cells and cells bearing the URA7 gene on a multicopy plasmid. The results of gel filtration chromatography indicated that the size of native CTP synthetase was consistent with a dimeric structure for the enzyme. CTP synthetase oligomerized to a tetramer in the presence of its substrates UTP and ATP. Maximum CTP synthetase activity was dependent on magnesium ions (4 mM) and 2-mercaptoethanol at the pH optimum of 8.0. CTP synthetase exhibited positive cooperative kinetics with respect to UTP and ATP and negative cooperative kinetics with respect to glutamine and GTP. CTP synthetase was potently inhibited by the product CTP which also increased the positive cooperativity of the enzyme toward UTP.(ABSTRACT TRUNCATED AT 250 WORDS)