A complex of iron and nucleic acid catabolites is a signal that triggers differentiation in a freshwater protozoan

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