Does poly-β-hydroxybutyrate interfere with nitrogen fixation in bacteroids?

While most rhizobia accumulate poly--hydroxybutyrate (PHB) during free-living reproductive growth, not all of them are able to store the polymer in symbiotic life. The role of PHB during nitrogen fixation in bacteroid state is still unclear. It has been reported that a PHB-synthase (phbC) mutant of Rhizobium etli has more nitrogenase activity and higher seed content (Cevallos et al., 1996). In contrast, bacteroids of Sinorhizobium meliloti do not accumulate the polymer and nitrogen fixation is not affected by the lack of PHB-synthase (Povolo et al., 1994). In S. meliloti a gene involved in carbon flux regulation (aniA) has been described. A mutation of this gene produces reduced nitrogen fixation suggesting a complex interplay among different metabolic pathways, namely PHB, glycogen and nitrogenase activity (Povolo and Casella, 2000). Recently a Tn5-mutant of Rhizobium tropici with enhanced symbiotic nitrogen fixation was isolated. The mutation was shown to map in a gene of glycogen syntheses (Marroqui et al., 2001). The aim of this work was to isolate and characterised R. tropici mutants unable to accumulate PHB.