FUSION OF SPIROPLASMA-FLORICOLA CELLS WITH SMALL UNILAMELLAR VESICLESIS DEPENDENT ON THE AGE OF THE CULTURE

Small unilamellar vesicles were labeled with the fluorescent probe oct adecylrhodamine B chloride and mixed with intact Spiroplasma floricola cells. The increase in fluorescence observed was interpreted as a res ult of the dilution of the probe in the unlabeled S. floricola membran es because of lipid mixing upon fusion. The progression of S. floricol a cultures to the stationary phase of growth was accompanied by a shar p decrease in the ability of the cells to fuse with small unilamellar vesicles. Low fusogenic activity was also detected in cells from cultu res that were aged in a growth medium maintained at pH 7.5 throughout the growth cycle. Chemical analysis of the cell membrane preparations isolated from cells harvested at the various phases of growth revealed that the phospholipid content and composition and the cholesterol/pho spholipid molar ratio were changed very little upon aging of the cultu res. Likewise, no changes in the fatty acid composition of membrane li pids were detected, with palmitic and oleic acids predominating throug hout the cycle. Nonetheless, upon aging of S. floricola cultures, a pr onounced increase in the levels of both cholesteryl esters, incorporat ed from the growth medium, and organic peroxides was observed. A decre ase in both fluorescence anisotropy of diphenylhexatriene and merocyan ine 540 binding to membranes of aged cells was also detected. The poss ible influence of these changes on the fusogenic activity of the cells is discussed.