Description:
<jats:p>Replacement of the normal culture liquid to a nitrate‐free medium resulted in an immediate drop in the ratio of protein to lipid in isolated cell envelopes of <jats:italic>Anacystis nidulans</jats:italic> cells. The relative fluidity of the envelope membranes or liposomes, made from the extracted lipids of the envelope, was estimated by measuring the steady‐state fluorescence polarization of 1,6‐diphenyl‐1,3,5‐hexatriene. A thermotrophic phase transition of lipids within the cytoplasmic membrane of intact cells was also revealed by detecting the temperature‐dependent absorption changes in the proportion of zeaxanthin at 390 nm. It became evident that a decrease in the proportion of protein to lipid within the cell envelope was accompanied neither by changes in the microviscosity level, nor by shifting of characteristic temperatures of the liquid‐crystalline‐to‐gel transition of lipids. In parallel with nitrate starvation, however, the proportion of saturated fatty acids of the envelope lipids increased markedly. Accumulation of saturated, longer‐chain (C<jats:sub>18</jats:sub>) fatty acids at the cost of C<jats:sub>16</jats:sub> counterparts upon nitrate deprivation occurred in all of the complex lipids. In accordance with these findings, a pronounced decrease in the fluidity was demonstrated for the liposomes prepared from the envelope polar lipids of nitrate‐starved cells compared with the corresponding control, throughout the temperature range (45–5°C) studied.</jats:p><jats:p>We propose that the fluidizing effect due to a fall in the ratio of protein to lipid was compensated by a rapidly triggering regulatory process which enables the preservation of the fluidity characteristics at an optimal level within the cell envelope of <jats:italic>A. nidulans</jats:italic>.</jats:p>