Fine structure of the cytomembranes of Nitrosocystis oceanus.

Thin-sectioned, negatively stained, and freeze-etched preparations of Nitrosocystis oceanus cytomembranes were compared. The cytomembranes in freeze-etched cells were covered with 80- to 120-A particles. When cells were disrupted and differentially centrifuged, various membrane and particle fractions were obtained. Negatively stained membrane fragments from the pellet centrifuged at 3,000 x g showed 70- to 80-A stalked particles, whereas those from the pellet centrifuged at 39,000 x g exhibited a crystalline array of subunits with a 30- to 40-A periodicity. High-speed supernatant and pellet fractions centrifuged at greater than 39,000 x g contained 40- to 120-A free particles but no membranes. In chemically fixed cells, 40-A particles were found embedded in the matrix of membranes. Results suggest that the larger 80- to 120-A particles are enzyme complexes, whereas the smaller 30- to 40-A particles represent a structural protein or a lipoprotein of the membrane.