Potentiel de biodégradation de corynébactéries isolées d'une zone polluée par les hydrocarbures du pétrole

Le degré de pollution engendré par l’effluent de la raffinerie de pétrole de Sidi Kacem (Maroc) a été estimé pendant l’année 1997. La détermination de l’oxygène dissous, de la demande chimique et biochimique en oxygène, ainsi que d’autres caractères physico-chimiques, a montré que l’effluent brut de la raffinerie est fortement pollué et que cette pollution se répercute sur la qualité des eaux de l’oued Rdom recevant cet effluent. Une collection de souches bactériennes capables de croître en présence du pétrole brut comme seule source de carbone a été isolée à partir de différentes stations de la zone polluée. L’identification des bactéries à Gram positif a été réalisée à l’aide de galeries d’identification. L’utilisation de la réaction en chaîne de la polymérase (PCR) et les Primers “ERIC” a montré que les bactéries isolées possèdent les séquences consensus inversées répétées. Ces séquences engendrent un profil en accord avec l’identification de quatre espèces appartenant au genre Corynebacterium : C. diphteriae, C. fascians, C. pseudotuberculosis, C. xerosis, d’une souche de Rhodoccocus equi et d’un Actynomyces. Le pourcentage de biodégradation du pétrole brut a été déterminé, il est de l’ordre de 40 % en quatre semaines pour les corynébactéries et la souche de Rhodoccocus et il est inférieur à 10 % pour la souche d’Actinomyces sp. Ces résultats sont en faveur d’un rôle de la flore à Gram positif dans la bioremédiation du site pollué

Nitrogen-fixing bacteria associated with leguminous and non-leguminous plants

Nitrogen is generally considered one of the major limiting nutrients in plant growth. The biological process responsible for reduction of molecular nitrogen into ammonia is referred to as nitrogen fixation. A wide diversity of nitrogen-fixing bacterial species belonging to most phyla of the Bacteria domain have the capacity to colonize the rhizosphere and to interact with plants. Leguminous and actinorhizal plants can obtain their nitrogen by association with rhizobia or Frankia via differentiation on their respective host plants of a specialized organ, the root nodule. Other symbiotic associations involve heterocystous cyanobacteria, while increasing numbers of nitrogen-fixing species have been identified as colonizing the root surface and, in some cases, the root interior of a variety of cereal crops and pasture grasses. Basic and advanced aspects of these associations are covered in this review

Modulation of NifA activity by PII in Azospirillum brasilense: evidence for a regulatory role of the NifA N-terminal domain.

Azospirillum brasilense NifA, which is synthesized under all physiological conditions, exists in an active or inactive from depending on the availability of ammonia. The activity also depends on the presence of PII, as NifA is inactive in a glnB mutant. To investigate further the mechanism that regulates NifA activity, several deletions of the nifA coding sequence covering the amino-terminal domain of NifA were constructed. The ability of these truncated NifA proteins to activate the nifH promoter in the absence or presence of ammonia was assayed in A. brasilense wild-type and mutant strains. Our results suggest that the N-terminal domain is not essential for NifA activity. This domain plays an inhibitory role which prevents NifA activity in the presence of ammonia. The truncated proteins were also able to restore nif gene expression to a glnB mutant, suggesting that PII is required to activate NifA by preventing the inhibitory effect of its N-terminal domain under conditions of nitrogen fixation. Low levels of nitrogenase activity in the presence of ammonia were also observed when the truncated gene was introduced into a strain devoid of the ADP-ribosylation control of nitrogenase. We propose a model for the regulation of NifA activity in A. brasilense

Functional analysis of the fixNOQP region of Azorhizobium caulinodans.

The deduced amino acid sequences of four open reading frames identified upstream of the fixGHI region in Azorhizobium caulinodans are very similar to the putative terminal oxidase complex coded by the fixNOQP operons from Rhizobium meliloti and Bradyrhizobium japonicum. The expression of the A. caulinodans fixNOQP genes, which was maximal under microaerobiosis, was positively regulated by FixK and independent of NifA. In contrast to the Fix- phenotype of B. japonicum and R. meliloti fixN mutants, an A. caulinodans fixNO-deleted mutant strain retained 50% of the nitrogenase activity of the wild type in the symbiotic state. In addition, the nitrogenase activity was scarcely reduced under free-living conditions. Analysis of membrane fractions of A. caulinodans wild-type and mutant strains suggests that the fixNOQP region encodes two proteins with covalently bound hemes, tentatively assigned to fixO and fixP. Spectral analysis showed a large decrease in the c-type cytochrome content of the fixN mutant compared with the wild type. These results provide evidence for the involvement of FixNOQP proteins in a respiratory process. The partial impairment in nitrogen fixation of the fixN mutant in planta may be due to the activity of an alternative terminal oxidase compensating for the loss of the oxidase complex encoded by fixNOQP