Cells of a genetic variant of the hamster fibroblast line BHK 21 which lack inosinic pyrophosphorylase activity (IPP- cells) and therefore cannot normally incorporate [3H]hypoxanthine were grown in mixed culture with cells of BHK 21 sublines which have inosinic pyrophosphorylase activity (IPP+ cells). If not in contact with IPP+ cells, IPP- cells do not incorporate added [3H]hypoxanthine into nucleic acid. IPP+ cells always do incorporate [3H]hypoxanthine and IPP- cells when in direct or indirect contact with IPP+ cells also incorporate the isotope. Cell to cell contact appears to be essential for this gain of a metabolic function by IPP- cells.
The possible molecular basis and general implications of the phenomenon are discussed.