The observation that phaC and phaB mutants of S. meliloti are still able to establish successful symbioses [24] suggests that synthesis of succinoglycan in these mutants, albeit

at a reduced level, Alvelestat in vitro is still sufficient to facilitate nodulation. This is consistent with previous reports which suggest that the production of small amounts of low-molecular-weight (LMW) EPS is sufficient to establish a successful symbiosis [29]. Indeed, it is conceivable that the competition defect observed in phaC mutants of S. meliloti may be due to extremely low levels of succinoglycan production. The phaC mutant may produce sufficient succinoglycan to establish an effective symbiosis but, assuming that the succinoglycan itself is playing a role in signalling during early nodulation, not enough to allow it to compete with strains producing higher levels of the EPS. Interestingly, the phaZ mutant demonstrates wild-type competitiveness and is able to out-compete both the phaC and bdhA mutants for nodulation. It is conceivable that another metabolic pathway that is dependent on D-3-HB metabolism may play a role in nodulation competitiveness. It is noteworthy that, although it has higher succinoglycan production than Rm1021, the phaZ mutant was not more competitive than the wild-type strain. While learn more it is tempting to speculate that there may be a critical level of succinoglycan, above which, further gains in competitiveness are not seen, further information regarding

the synthesis of succinoglycan during the infection process is still needed. Studies are currently underway in our lab to investigate this possibility further. It is conceivable that, when PHB synthesis is inhibited,

intermediates required Cyclooxygenase (COX) for succinoglycan are not synthesized efficiently. It is also possible that, in the absence of a functional PHB synthesis pathway, enzymes required for succinoglycan may be inhibited or down-regulated. Furthermore, it has been suggested that acetyl phosphate may provide a regulatory link between PHB and succinoglycan synthesis [30]. Studies in the thermophilic cyanobacterium Synechococcus sp. strain MA19, have shown that acetyl phosphate is involved in the post-translational regulation of PHB synthase in vitro, and that this regulation is concentration-dependent [30]. As well, that study revealed that the enzyme phosphotransacetylase, which converts acetyl-CoA to acetyl phosphate, is only active under PHB-accumulating conditions [30]. In E. coli, acetyl phosphate is known to act as a global signal which acts through two-component regulatory signals [31], perhaps by direct phosphorylation of the response regulator [32] itself. Furthermore, the ChvI protein, of the S. meliloti ExoS-ChvI two-component regulatory system, is able to autophosphorylate in the presence of acetyl phosphate in vitro [33]. Since PHB synthesis mutants may excrete excess acetyl-CoA, levels of acetyl phosphate will likely be low under these conditions.