Leakage of periplasmic enzymes from lipopolysaccharide-defective mutants of Salmonella typhimurium

Abstract

Mutants of Salmonella typhimurium with defects in the heptose region of the lipopolysaccharide (LPS) molecule (heptose-deficient, chemotype Re) leak periplasmic enzymes (acid phosphatase (EC 3.1.3.2), cyclic phosphodiesterase, ribonuclease I (EC 3.1.4.22), and phosphoglucose isomerase (EC 5.3.1.9) (PGI is at least partially periplasmic in E. coli and S. typhimurium; see below)) and do not leak an internal enzyme (glucose-6-phosphate dehydrogenase) into the growth medium. The extent of this leakage is markedly increased at higher temperature (42 °C). Leakage of periplasmic enzymes from the strains lacking units distal to heptose I in the LPS molecule (chemotype Rd₂) occurs only at 42 °C, and not at 30 or 37 °C. The extent of leakage of these enzymes from smooth strain and mutants of other LPS chemotypes (Rc, Rd₁) is not significant, and is not influenced by growth temperatures. The kinetics of leakage of periplasmic enzymes after shift to 42 °C in nutrient broth reveal an accelerated release into the medium from heptose-deficient strains of cyclic phosphodiesterase and ribonuclease I after 30 min at 42 °C, and phosphoglucose isomerase after 60 min at 42 °C; at 30 °C the rate of release of cyclic phosphodiesterase and ribonuclease I is relatively slower. After 60 min at 42 °C in nutrient broth, growth of these strains has either slowed down or stopped. In L-broth, which permits the growth of the heptose-deficient strain (SA1377) at 42 °C, leakage of cyclic phosphodiesterase and phosphoglucose isomerase occurs, whereas there is no detectable leakage of these enzymes from the isogenic smooth strain (SA 1355). Thus, leakage of the periplasmic enzymes from the heptose-deficient strain occurs with or without growth. Mg²⁺ (0.75 mM), sodium chloride (50 mM), and sucrose (100 mM) in nutrient broth at 42 °C prevent the leakage of these enzymes. The shedding of LPS from the heptose-deficient as well as the smooth strains is enhanced by high temperature (42 °C), whereas considerable leakage of protein occurs only in the heptose-deficient strain at 42 °C and not in the smooth strain. The smooth and heptose-deficient strains are equally sensitive to osmotic shock although a significant proportion of acid phosphatase and cyclic phosphodiesterase activities from the heptose-deficient cells grown at 42 °C comes off in the Tris-NaCl wash step suggesting a rather loose attachment of these enzymes onto the cell surface.