Apoptosis as a form of programmed cell death (PCD) in multicellular organisms is a well-established genetically controlled process that leads to elimination of unnecessary or damaged cells. Recently, PCD has also been described for unicellular organisms as a process for the socially advantageous regulation of cell survival. The human Bcl-2 family member Bak induces apoptosis in mammalian cells which is counteracted by the Bcl-x L protein. We show that Bak also kills the unicellular fission yeast Schizosaccharomyces pombe and that this is inhibited by coexpression of human Bcl-x L . Moreover, the same critical BH3 domain of Bak that is required for induction of apoptosis in mammalian cells is also required for inducing death in yeast. This suggests that Bak kills mammalian and yeast cells by similar mechanisms. The phenotype of the Bak-induced death in yeast involves condensation and fragmentation of the chromatin as well as dissolution of the nuclear envelope, all of which are features of mammalian apoptosis. These data suggest that the evolutionarily conserved metazoan PCD pathway is also present in unicellular yeast.Programmed cell death (PCD) in metazoans is an essential homeostatic mechanism permitting the removal of surplus cells during morphogenesis and tissue maintenance and the deletion of cells that present a risk to the organism because they are mutated or infected (10,19,35,36,40). For vertebrates, the descriptive name commonly given to the process of PCD is apoptosis. Classical apoptosis is characterized by membrane blebbing, cell shrinkage, chromatin condensation, and nuclear and cellular fragmentation, and it results from the activation of an intrinsic suicide program (47). Recent studies implicate the dysregulation of PCD in the pathophysiology of several human diseases, including AIDS (12, 28), neurodegenerative disease (25,37,44), and cancer (for a review, see reference 42).The basal machinery responsible for metazoan PCD is highly evolutionarily conserved and, at its execution level, involves the action of a discrete class of cysteine proteases, of which the prototypes are the interleukin-1␤-converting enzyme in humans and Ced-3 in the nematode (49). Also conserved are key regulators of apoptosis: in Caenorhabditis elegans the Ced-9 protein and in humans the Bcl-2 protein family (18, 46), which comprises both suppressors (e.g., Bcl-2 and Bcl-x L ) and promoters (e.g., Bax and Bak) of PCD (17,30,46).Recently, there have been several reports describing apparent PCD in the unicellular eukaryotes Tetrahymena thermophila, Dictyostelium discoideum, Trypanosoma brucei rhodsiense, and Trypanosoma cruzi and even in bacteria (2, 6, 45, 48; for an overview, see reference 1). PCD in unicellular organisms might facilitate constant selection for the fittest cell in the colony or optimal adaptation of cell numbers to the environment or might serve as a means for altruistic cell death to prevent the spread of virus in the event of infection.It has been shown that expression of the mammalian Bax protein in th...

In fission yeast, passage through START and into S-phase requires cyclin-dependent kinase (CDK) activity and the periodic transcription of genes essential for S-phase ('S-phase transcription'). Here we investigate the control of this transcription in the mitotic cell cycle. We demonstrate that the periodicity of S-phase transcription is likely to be controlled independently of CDK activity. This contrasts with the equivalent system in budding yeast. Furthermore, the CDK function required for S-phase acts after the onset of S-phase transcription and after the accumulation of cdc18p, a critical target of this transcriptional machinery. We investigate the role of individual components of the S-phase transcriptional machinery, cdc10p, res1p, res2p and rep2p, and define a new role for res2p, previously demonstrated to be important in the meiotic cycle, in switching off S-phase transcription during G2 of the mitotic cycle. We show that the presence of the in vitro bandshift activity DSC1, conventionally thought to represent the active complex, requires res2p and correlates with inactive transcription. We suggest that S-phase transcription is controlled by both activation and repression, and that res2p represses transcription in G2 of the cell cycle as a part of the DSC1 complex.

In fission yeast, cdc18p plays a critical role in bringing about the onset of S phase. We show that cdc18p expression is subject to a complex sequence of cell cycle controls which ensure that cdc18p levels rise dramatically as cells exit mitosis, before the appearance of CDK activity in G 1 . We find that transcription of cdc18, together with the transcription of other cdc10p/ res1p targets, is first initiated as cells enter mitosis and continues even in cells arrested in mitosis with highly condensed chromatin. However, cdc18p cannot accumulate during mitosis because it is targeted for proteolysis by mitotic cdc2p-protein kinase-mediated phosphorylation. On exit from mitosis, the cdc2p mitotic kinase activity falls, stabilizing cdc18p, which then rapidly accumulates. This combination of mitotic transcription and CDK-mediated proteolysis ensures that progression through mitosis simultaneously prepares cells for DNA replication. During S phase, cdc18 transcription is then switched off, preventing the reinitiation of DNA synthesis until the completion of the next round of mitosis.

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Non-genotoxic hepatocarcinogenesis may involve suppression of the hepatocyte apoptosis that would normally remove damaged or initiated cells. These protected hepatocytes could then remain as preferential targets for promotion by this class of compounds. Here, we demonstrate clearly that the non-genotoxic liver carcinogens and hepatomitogens cyproterone acetate (CPA) and nafenopin, a peroxisome proliferator, both suppressed the basal level of rat liver apoptosis in vivo to 17 or 77% of controls, respectively. Concomitant with this suppression of apoptosis, BrdU labelling indices and mitotic figures rose confirming a perturbation of both sides of the growth equation between cell death and replication. Withdrawal of CPA or nafenopin resulted in a 100 to 200fold elevation in apoptosis. Tlus was idubited by the readnunstration of either compound. generated upon withdrawal of CPA or nafenopin. Rats were administered BrdU during the hyperplastic phase of compound administration (0-10 days). Livers were examined 5 days after compound withdrawal. With both CPA and nafenopin, apoptotic b d e s and S-phase were predominantly in the periportal region. However, despite this zonal co-localisation , very few (