Bacteroides symbiosis

D-Fructose 1-phosphate kinase catalyzes the phosphorylation of D-fructose 1-phosphate to D-fructose 1,6-bisphosphate. The enzyme has been purified from Aerobacter aerogenes326 and Bacteroides symbiosis.327... 

A phosphorylated form of the enzyme was isolated (Milner and Wood, 1972) after an incubation of the enzyme with P-PEP. Evans and Wood (1968) isolated a pyrophosphorylated form of the enzyme and demonstrated experimentally a ping-pong mechanism of the dikinase reaction. It has been suggested that various dikinases play an important role in gluconeogenesis (Slack and Hatch, 1967 Reeves et al., 1968), since they are foimd in parasitic amebae and Bacteroides symbiosis, which lack the ubiquitous pyruvate kinase (Reeves et al, 1968). The dikinase reaction is shifted to the right by the utilization of PEP for biosynthesis and by the hydrolysis of PPi formed in the reaction by pyrophosphatases. 

Nickel availability to the host plants severely limits the expression of the R. leguminosarum hydrogenase genes in the P. sativum symbiosis (Brito et al. 1994) and probably in other symbioses such as M. loti-L. corniculatus (Brito et al. 2000). This limitation occurs at the level of processing of the enzyme subunits (Brito et al. 1994). It is not clear, however, whether Ni limitation is due to the bacterial or the plant component of the symbiosis. Recent results of nickel transport experiments with intact pea symbiosomes indicate that the peribacteroid membrane is not a specific barrier for Ni transport into the bacteroid (Bascones et al. unpublished). 

In symbiosis with Fabales, bacteria live as bacteroids in root nodules inside the plant cells. The plant supplies the bacteroids with nutrients, but it also benefits from the fixed nitrogen that the symbionts make available. 

Mellor, R. (1989). Bacteroids in the Rhizobium- egume symbiosis inhabit a plant internal lytic compartment Implications for other microbial endosymbioses. Journal of Experimental Botany 40, 831-9. 

Sinorhizobium meliloti could accumulate PHB during fiee life, but not in symbiosis (Figure 1). It was hypothesised that this could be due to a low activity of the NADH-dependent malic enzyme . S. meliloti has two malic enzymes, one NADH dependent (Dme) and the other NADPH dependent (Tme). While Dme and Tme are both expressed in the fiee-living state, Tme is repressed in bacteroids and Dme is inhibited by excess of acetyl-CoA. As a consequence PHB is not accumulated in the bacteroid because die levels of NAD(P)H are too low. 

The preceding description of nodule initiation, development, and nitrogen-ase clearly demonstrates that the host contribution to symbiosis can be grouped into several functions, including recognition, root hair invasion, infection thread formation, nodule differentiation (e.g., meristems, vascular elements, sclerenchyma), carbon assimilation, organic acid production, ammonia assimilation, maintenance of low O2 by Lb, separation of bacteroids and host cytoplasm by the PBM, senescence, and possibly suppression of host defense responses required for nodule compatibility and/or effectiveness. 

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