Osmotic Regulation and the Biosynthesis of Membrane-Derived Oligosaccharides in Escherichia coli

The membrane-derived oligosaccharides (MDO) of Escherichia coli are periplasmic constituents containing 8--10 glucose units in a highly branched structure, linked by β 1-2 and β 1--6 bonds [Schneider, J. E., Reinhold, V., Rumley, M. K. & Kennedy, E. P. (1979) J. Biol. Chem. 254, 10135--10138]. The MDO are multiply substituted with sn-1-phosphoglycerol residues (derived from membrane phosphatidylglycerol) and with O-succinyl ester residues and, thus, are highly anionic. Experiments in this paper offer evidence that the biosynthesis of MDO is an important aspect of osmoregulation in E. coli. Cells grown in medium of low osmolarity (ca. 50 mosM) synthesize 16 times more MDO than those grown in the same medium with 0.4 M NaCl. In cells grown in medium of low osmolarity, it appears that MDO is the principal source of fixed anion in the periplasmic space and, thus, acts to maintain the high osmotic pressure and Donnan membrane potential of the periplasmic compartment. Regulation of MDO synthesis in response to changes in osmolarity of the medium appears to occur at the genetic level because the synthesis of new protein is needed to permit the production of MDO at high rates after shift of cells to medium of low osmolarity.