The polymorphic ciliated protozoan Tetrahymena vorax can undergo differentiation from the microstomal form, which normally feeds on bacteria and other particulate matter, into the macrostomal cell type, which is capable of ingesting prey ciliates. The process is triggered by exposure of the microstome to an inducer contained in stomatin, an exudate of the prey. To establish the identity of the signal, stomatin was fractionated by combinations of cation exchange, HPLC, and TLC, and the fractions were assayed for biological activity. Although no single active fraction of purified inducer was obtained, all fractions with activity contained ferrous iron and the nucleic acid catabolites hypoxanthine (6-oxypurine) and uracil (2,4-dioxopyrimidine), probably in a chelated form. The activity of synthetic complexes containing these three components is equivalent to stomatin. These results indicate a role for ferrous iron and its potential in chelated form to signal differentiation in certain protozoa and, perhaps, in other organisms as well.

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ABSTRACT. After formation, food vacuoles in the microstomal cell type of Tetrahymena vorax labeled with either India ink or cationized ferritin (CF) undergo three morphological stages during processing: an initial condensation with a size decrease, the appearance of a clear halo around the condensed marker that apparently results from expansion of the vacuole, and the disappearance of the halo region. These stages correspond generally to three of the four stages described previously for Tetrahymena. Fusion of older vacuoles prior to egestion may represent a fourth stage in vacuole processing. The developing vacuole has small coated regions of the membrane; two populations of vesicles, small coated vesicles and larger vesicles with electron‐dense cores, are adjacent to it. During the initial condensation, tubular invaginations with CF project from the vacuole. After the disappearance of the halo, much of the vacuolar surface appears to be multilayered with extensive folds containing CF. Labeled flattened or curved disks and closed rings occur in the cytoplasm often adjacent to labeled vacuoles and in the vicinity of the cytoproct. The appearance of labeled disks and rings is identical to both labeled and unlabeled vesicles adjacent to the cytopharynx, adding additional support to the hypothesis that, like Paramecium, vesicles retrieved from food vacuoles during processing as well as after fusion with the cytoproct may be reused for vacuole formation.

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