A Study of the Cytoplasmic Membrane of the Cyanobacterium Synechococcus PCC 7942 Cells Adapted to Different Nitrogen Sources By Laser Doppler Electrophoresis

Plasmalemma surface represents the first sensor of environment changes. Among all its responses, the plasmalemma protein composition reflects the physiological processes involved in adaptation. This is easily perceived on the different polypeptide compositions of the SynechococcusPCC 7942 cytoplasmic membrane from cells adapted to nitrate or ammonium. Substitution of ammonium by nitrate in the growth medium stimulates the occurence of 45 kDa [1—3] and 126 kDa [4] proteins. The former has been already described as the nitrate binding protein from experiments carried out on a soluble truncated form [5]. The amount of these proteins is 25% [3] and 2.5% [4] of the plasmalemma proteins, respectively, in low nitrate content medium. They are absent in plasmalemma from ammonium grown cells. The amplitude of the expression-repression processes is large enough to result in surface charge density variations. Laser Doppler electrophoresis measurements are particularly designed to provide informations not only on the surface charge variations related to the adaptation process but also on the interactions between the membrane surface and ions [6—8]. Our approach in this paper is to establish the differentiation of the adapted membranes and to shed light on the nitrate ions interactions at a greater level of organisation than a modified protein. For the purpose of electrokinetic properties comparison, three conditions of adaptation were selected i.e.2 mM or 175 mM nitrate and 5 mM ammonium. The vesicles obtained from these membranes were compared with regard to their interactions with different salts.