Physiology of the cell surface of Neurospora ascospores. II. Interference with dye adsorption by polymyxin

Abstract

Polymyxin B prevents the germination of ascospores of Neurospora tetrasperma with an LD50 of 3-4 p.p.m. The toxic effect of polymyxin is partially reversed by calcium and magnesium ions. The effect of polymyxin on the respiration of activated ascospores does not become apparent until after 2 hr. The effect upon respiration is reversed by calcium ions. Dormant and activated ascospores remove about the same amount of polymyxin from solution almost immediately upon exposure to the antibiotic. After 90 min. the apparent uptake of activated cells is markedly diminished while that of the dormant spores increases slightly. Polymyxin competes noncompetitively with methylene blue for sites on the surface of the cells when added simultaneously with the dye. However, if the antibiotic is added before methylene blue, dye uptake is almost entirely suppressed and an uncompetitive type of inhibition results. These data, in conjunction with those showing polymyxin uptake by cell-wall fragments, suggest that the antibiotic occupies sites on the surface of the ascospore and that these sites differ from those binding methylene blue.