Rebeca Nájera

Characterization of the Azotobacter vinelandii algC gene involved in alginate and lipopolysaccharide production

Azotobacter vinelandii is a soil gamma-proteobacteria that fixes nitrogen and forms desiccation-resistant cysts. The exopolysaccharide alginate is an integral part of the layers surrounding the cysts. Here, we reported the cloning of A. vinelandii algC, encoding the enzyme catalyzing the second step of alginate pathway. We showed that AlgC is involved not only in alginate production, but also in lipopolysaccharide (LPS) synthesis and that it seems to have both phosphomannomutase and phosphoglucomutase activities. The transcriptional analysis of the A. vinelandii algC gene showed that it contained two start sites, one of which was dependent on the alternative sigma factor AlgU/AlgT. This finding explains why alginate biosynthesis is dependent on AlgU activity, since all other alginate biosynthetic genes have been characterized previously and algC is the only alginate structural gene that is directly transcribed by this sigma factor.

Formation of Rhizobium phaseoli symbiotic plasmids by genetic recombination.

We report here the formation of symbiotic plasmids (pSyms), by genetic recombination between rearranged pSyms, which lack symbiotic information, and resistance plasmids carrying parts of different symbiotic plasmids (Rs). This recombination was found to occur both between plasmids derived from different Rhizobium phaseoli isolates, and between plasmids derived from strains obtained from the same original isolate.

Partial deletion of the Rhizobium phaseoli CFN23 symbiotic plasmid implies a concomitant amplification of plasmid DNA sequences.

Rhizobium leguminosarum biovar phaseol CFN23 loses its ability to nodulate beans at a high frequency because of a deletion of part of its symbiotic (pSym) plasmid (Soberón‐Chávez et al., 1986). We report here that at least 80kb of pSym are deleted upon loss of the symbiotic phenotype; the deletion removes the nitrogenase structural nifHDK and the common nod ABC genes. The size of the deleted pSym is not reduced, since it is accompanied by an amplification of other pSym plasmid sequences. This genetic rearrangement is similar to the deletion and amplification of yeast mitochondrial DNA leading to‘petite’mutations.

Characterization of the gene involved in alginate and lipopolysaccharide production

Azotobacter vinelandii is a soil γ-proteobacteria that fixes nitrogen and forms desiccation-resistant cysts. The exopolysaccharide alginate is an integral part of the layers surrounding the cysts. Here, we reported the cloning of A. vinelandii algC, encoding the enzyme catalyzing the second step of alginate pathway. We showed that AlgC is involved not only in alginate production, but also in lipopolysaccharide (LPS) synthesis and that it seems to have both phosphomannomutase and phosphoglucomutase activities. The transcriptional analysis of the A. vinelandii algC gene showed that it contained two start sites, one of which was dependent on the alternative sigma factor AlgU/AlgT. This finding explains why alginate biosynthesis is dependent on AlgU activity, since all other alginate biosynthetic genes have been characterized previously and algC is the only alginate structural gene that is directly transcribed by this sigma factor.