Programmed cell death (PCD), also referred to as apoptosis, is a cellular "suicide" mechanism, based on information from its own internal metabolism, environment, developmental history, and genome. This system was described in eukaryotes continuously along evolution, through amoebae, nematodes, insects, and animals. PCD is essential for the proper development or function of a cell system, organ, or survival of the organism as a whole. Research in the last 2 decades has shown that the life cycle of several prokaryotic organisms display developmental programs, similar to metazoan differentiation, that is part of their adaptation to stressful environments. These include warmer cell formation and differentiation in Caulobacter cereus, sporulation in Bacillus and Streptomyces, heterocyst formation in Anabaena, development of bacteroids in Rhizobium, the formation of multicellular fruiting bodies and sporulation in Myxobacteria, and the formation of nonculturable, but viable, cells in various Gram-negative bacteria. Moreover, and more significantly, the photosynthetic bacteria Rhodobacter capsulatus were shown to release nucleoprotein particles designated "gene transfer agent (GTA)" as they enter the stationary phase. GTAs contain DNA of 3.6 x 10(6) molecular weight, representing all parts of the genome, and they may be taken up by other strains of R. capsulatus, and complement mutants. We postulate that these various modes of stress adaptations in bacteria are prokaryotic manifestation, and possibly the phylogenetic precursor, of the eukaryotic phenomenon, programmed cell death, and therefore we propose to designate it "proapoptosis". In addition to their function, apoptosis and proapoptosis share various mechanistic programmed features, including DNA fragmentation and packaging, cell shrinkage, degradation of RNA, proteolysis and synthesis of new proteins, and the involvement of reactive oxygen species.