On the relationship of intracytoplasmic to cytoplasmic membranes in nitrogen-fixing Azotobacter vinelandii

Abstract

Dinitrogen fixing cells of Azotobacter vinelandii increase the ratio of intracytoplasmic to cytoplasmic membranes surface area linearly by a factor of six when growing in an oxygen controlled chemostat from about 1% to 100% air saturation of the culture medium (Post et al. 1982). In spite of this, cells grown at 1%, 45%, and 80% air saturation exhibited only a single membrane fraction with a buoyant density of ϱ20 = 1.176 g cm−3 after equilibrium sucrose density gradient centrifugation. (Cell wall exhibited a density of ϱ20 = 1.23 g cm−3.) Total cell membrane preparations were employed to study the proportion of respiratory reactions with cells grown at various oxygen concentrations. Maximum activities as well as substrate affinities of NADH, NADPH, and malate dehydrogenases increased from about 1% to 30% air saturation and stayed almost constant at higher oxygen concentrations. Activities of the three dehydrogenases changed largely in parallel. Essentially the same dependency on oxygen concentration was measured with the rate limiting step of the terminal respiratory pathway. Consequently, the ratio of the dehydrogenase activities to the activity of the rate limiting step of the terminal pathway stayed constant. The data suggest that intracytoplasmic and cytoplasmic membranes do not differ with respect to respiratory activities. It is discussed that both types of membranes represent differently localized parts of an otherwise identical membrane system.