Phosphorus-free membrane lipids of Sinorhizobium meliloti are not required for the symbiosis with alfalfa but contribute to increased cell yields under phosphorus-limiting conditions of growth

The microsymbiont of alfalfa, Sinorhizobium meliloti, possesses phosphatidylglycerol, cardiolipin, phosphatidylethanolamine, and phosphatidylcholine as major membrane phospholipids, when grown in the presence of sufficient accessible phosphorus sources. Under phosphate-limiting conditions of growth, S. meliloti replaces its phospholipids by membrane lipids that do not contain any phosphorus in their molecular structure and, in S. meliloti, these phosphorus-free membrane lipids are sulphoquinovosyl diacylglycerols (SL), ornithine-containing lipids (OL), and diacylglyceryl-N,N,N-trimethylhomoserines (DGTS). In earlier work, we demonstrated that neither SL nor OL are required for establishing a nitrogen-fixing root nodule symbiosis with alfalfa. We now report the identification of the two structural genes btaA and btaB from S. meliloti required for DGTS biosynthesis. When the sinorhizobial btaA and btaB genes are expressed in Escherichia coli, they cause the formation of DGTS in this latter organism. A btaA-deficient mutant of S. meliloti is unable to form DGTS but can form nitrogen-fixing root nodules on alfalfa, demonstrating that sinorhizobial DGTS is not required for establishing a successful symbiosis with the host plant. Even a triple mutant of S. meliloti, unable to form any of the phosphorus-free membrane lipids SL, OL, or DGTS is equally competitive for nodule occupancy as the wild type. Only under growth-limiting concentrations of phosphate in culture media did mutants that could form neither OL nor DGTS grow to lesser cell densities.     

Intracellular Ca2+ transients in delta-sarcoglycan knockout mouse skeletal muscle

Background

δ-Sarcoglycan (δ-SG) knockout (KO) mice develop skeletal muscle histopathological alterations similar to those in humans with limb muscular dystrophy. Membrane fragility and increased Ca²⁺ permeability have been linked to muscle degeneration. However, little is known about the mechanisms by which genetic defects lead to disease.

Methods

Isolated skeletal muscle fibers of wild-type and δ-SG KO mice were used to investigate whether the absence of δ-SG alters the increase in intracellular Ca²⁺ during single twitches and tetani or during repeated stimulation. Immunolabeling, electrical field stimulation and Ca²⁺ transient recording techniques with fluorescent indicators were used.

Results

Ca²⁺ transients during single twitches and tetani generated by muscle fibers of δ-SG KO mice are similar to those of wild-type mice, but their amplitude is greatly decreased during protracted stimulation in KO compared to wild-type fibers. This impairment is independent of extracellular Ca²⁺ and is mimicked in wild-type fibers by blocking store-operated calcium channels with 2-aminoethoxydiphenyl borate (2-APB). Also, immunolabeling indicates the localization of a δ-SG isoform in the sarcoplasmic reticulum of the isolated skeletal muscle fibers of wild-type animals, which may be related to the functional differences between wild-type and KO muscles.

Conclusions

δ-SG has a role in calcium homeostasis in skeletal muscle fibers.

General significance

These results support a possible role of δ-SG on calcium homeostasis. The alterations caused by the absence of δ-SG may be related to the pathogenesis of muscular dystrophy.