Fractionation and Characterization of the Plasma and Mesosome Membrane of Listeria monocytogenes1

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Protoplasts of Listeria monocytogenes strain 42 were fractionated after control lysis on a Ficoll (a polysucrose) density gradient. Visually, five zones could be recognized in the gradient. The first one was composed of amorphous cytoplasmic solutes (fraction 1a) and a mixture of particles (fraction 1b). These were: (i) light particles that were lipase-sensitive and composed of six subunits and (ii) heavy particles, sensitive to ribonuclease and devoid of fine structure. The second zone consisted of tubules and vesicles still harboring cytoplasmic components (fraction 2), whereas the third zone contained only empty vesicles and protoplast ghosts (fraction 3). The material congregating into the fourth zone was morphologically identical to that of the third (fraction 3a). The fifth and heaviest zone contained a mixture of (i) particles without any substructure and (ii) partly lysed protoplasts (fraction 4). Fractions 1b and 4 were the richest in nucleic acids (ribonucleic acid, 11.4 and 9.4%, respectively; deoxyribonucleic acid, 5.1 and 4.8%, respectively), whereas fraction 1b had the highest protein contents (74.6%). Phospholipids were mainly found in fractions 2 and 3. Except for fraction 1, all materials contained significant amounts of protein-bound phosphorus. The main concentrations of four enzymes were: glucose-6-phosphate dehydrogenase (fraction 1a); adenosine triphosphatase and reduced nicotinamide adenine diphosphate oxidase (fraction 3); nitro blue tetrazolium chloride reductase (fraction 2). Fractionation of strain 42 after addition of 32P during the mid-log phase of growth revealed that the radio-activity was mainly detected in fraction 1b, when growth in the presence of the marker was allowed for 10 min, and in fraction 2, when growth was allowed for 90 min. The vesicles of fraction 2, often tubular, are probably of mesosomal origin, whereas those of fraction 3, which are always spherical, represent, most likely, the bulk of the cell plasma membrane. Our data showed slight chemical differences between these two fractions, but the differences in enzymatic activities and lipid-phosphorus incorporation during long pulse experiments were most dramatic.

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