Fracture-temperature related differences in the ultrastructure of plasmalemma P faces of freeze-fractured baker's yeast (Saccharomyces cerevisiae) have been observed in high-resolution replicas prepared in freeze-etch systems pumped to 2 X 10(-7) torr in which the specimens were protected from contamination by use of liquid nitrogen-cooled shrouds. Two major P-face images were observed regardless of the source of the yeast, the age of the culture, the growth temperature, the physiological condition, or the suspending medium used: (a) a "cold-fracture image" with many strands closely associuated with tubelike particles (essentially the same image as those previously published for yeast freeze-fractured at 77 degrees K), and (b) a "prefracture image" characterized by the presence of more distinct tubelike particles with few or no associated strands (for aging cultures, the image recently referred to as "paracrystalline arrays" of "craterlike particles"). Both types of P-face image can be found in separate areas of single replicas and occasionally even within a single plasma membrane. Whereas portions of replicas known to be fractured at any temperature colder than 218 degrees K reveal only the cold-fracture image, prefracture images are found in cells intentionally fractured at 243 degrees K and in cracks or fissures which develop during the freezing of other specimens. These findings demonstrate that the prefracture image results from the fracturing of specimens at some temperature above 230 degrees K, no t from fracturing specimens at some temperature between 173 degrees and 77 degrees K, and not from the use of "starved" yeast cells.

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