Vigorous apoptosis is induced 3–4 hours after activation of cAMP-dependent protein kinase I in the rat myeloid leukemia cell line IPC-81 [J. Cell. Physiol. 146 (1991) 73-80]. We will report a novel feature of apoptosis in these cells: a selective and temporarily ordered cleavage within the two largest 28S ribosomal RNA variable regions (V3 and V13). The cleavage of 28S rRNA coincided with internucleosomal DNA fragmentation and cessation of cellular protein synthesis. The implication of 28S variable regions as targets in apoptosis is a clue to the function of these so far apparently superfluous parts of eukaryotic ribosomes.
