Photosynthetic Bacterium Rhodobacter Capsulatus Research Articles

Evidence for a functional similarity between the two-component regulatory systems RegSR, ActSR, and RegBA (PrrBA) in alpha-Proteobacteria.

The symbiotic bacteria Bradyrhizobium japonicum and Sinorhizobium meliloti, and the purple photosynthetic bacteria Rhodobacter capsulatus, Rhodovulum sulfidophilum, Roseobacter denitrificans and Rhodobacter sphaeroides possess homologous two-component regulatory systems, namely RegSR, ActSR, RegBA and PrrBA. The respective response regulators of these bacteria control expression of different regulons that are involved in N2 fixation, CO2 fixation, photosynthesis or acid tolerance. We therefore asked whether the regulators are functionally exchangeable or whether they have disparate functions in the different species, despite the amino acid sequence similarity. In this study, we showed that purified B. japonicum RegR bound in vitro to genuine DNA targets for Rba. capsulatus RegA, and that RegA was phosphorylated in vitro when RegSc (a soluble variant of the sensor kinase RegS) was added to an Escherichia coli extract containing overexpressed RegA. In vivo, RegA and S. meliloti ActR activated transcription of the B. japonicum fixR-nifA operon, normally a target for RegR. The genes for both regulators, regA and actR, were able to complement a B. japonicum regR mutant with respect to the formation of a nitrogen-fixing symbiosis with soybean. Vice versa, RegR activated in Rba. capsulatus the expression of the photosynthesis operon puc, normally a target for RegA. In conclusion, the results show that B. japonicum RegR, Rba. capsulatus RegA, and S. meliloti ActR are functionally similar.

Reconstitution of Light-independent Protochlorophyllide Reductase from Purified Bchl and BchN-BchB Subunits: IN VITRO CONFIRMATION OF NITROGENASE-LIKE FEATURES OF A BACTERIOCHLOROPHYLL BIOSYNTHESIS ENZYME

Protochlorophyllide reductase catalyzes the reductive formation of chlorophyllide from protochlorophyllide during biosynthesis of chlorophylls and bacteriochlorophylls. The light-independent (dark) form of protochlorophyllide reductase plays a key role in the ability of gymnosperms, algae, and photosynthetic bacteria to green (form chlorophyll) in the dark. Genetic and sequence analyses have indicated that dark protochlorophyllide reductase consists of three protein subunits that exhibit significant sequence similarity to the three subunits of nitrogenase, which catalyzes the reductive formation of ammonia from dinitrogen. However, unlike the well characterized features of nitrogenase, there has been no previous biochemical characterization of dark protochlorophyllide reductase. In this study, we report the first reproducible demonstration of dark protochlorophyllide reductase activity from purified protein subunits that were isolated from the purple nonsulfur photosynthetic bacterium Rhodobacter capsulatus. Two of the three subunits (Bchl and BchN) were expressed in R. capsulatus as S tag fusion proteins that facilitated affinity purification. The third subunit (BchB) was co-purified with the BchN protein indicating that BchN and BchB proteins form a tight complex. Dark protochlorophyllide reductase activity was shown to be dependent on the presence of all three subunits, ATP, and the reductant dithionite. The similarity of dark protochlorophyllide reductase to nitrogenase is discussed.

Diphenylene iodonium as an inhibitor for the hydrogenase complex of Rhodobacter capsulatus. Evidence for two distinct electron donor sites

The photosynthetic bacterium Rhodobacter capsulatus synthesises a membrane-bound [NiFe] hydrogenase encoded by the H 2 uptake hydrogenase (hup)SLC structural operon. The hupS and hupL genes encode the small and large subunits of hydrogenase, respectively; hupC encodes a membrane electron carrier protein which may be considered as the third subunit of the uptake hydrogenase. In Wolinella succinogenes, the hydC gene, homologous to hupC, has been shown to encode a low potential cytochrome b which mediates electron transfer from H 2 to the quinone pool of the bacterial membrane. In whole cells of R. capsulatus or intact membrane preparation of the wild type strain B10, methylene blue but not benzyl viologen can be used as acceptor of the electrons donated by H 2 to hydrogenase; on the other hand, membranes of B10 treated with Triton X-100 or whole cells of a HupC − mutant exhibit both benzyl viologen and methylene blue reductase activities. We report the effect of diphenylene iodonium (Ph 2I), a known inhibitor of mitochondrial complex I and of various monooxygenases on R. capsulatus hydrogenase activity. With H 2 as electron donor, Ph 2I inhibited partially the methylene blue reductase activity in an uncompetitive manner, and totally benzyl viologen reductase activity in a competitive manner. Furthermore, with benzyl viologen as electron acceptor, Ph 2I increased dramatically the observed lagtime for dye reduction. These results suggest that two different sites exist on the electron donor side of the membrane-bound [NiFe] hydrogenase of R. capsulatus, both located on the small subunit. A low redox potential site which reduces benzyl viologen, binds Ph 2I and could be located on the distal [Fe 4S 4] cluster. A higher redox potential site which can reduce methylene blue in vitro could be connected to the high potential [Fe 3S 4] cluster and freely accessible from the periplasm.

Light intensity distribution in the externally illuminated cylindrical photo-bioreactor and its application to hydrogen production by Rhodobacter capsulatus

The light distribution in the externally illuminated cylindrical photo-bioreactor for production of hydrogen by a photosynthetic bacterium Rhodobacter capsulatus ST-410 was estimated. The estimation was performed on the basis of the Matsuura and Smith’s diffuse model [1]. In the diffuse model, the incident light rays are assumed to proceed in every direction and the local intensity is calculated as the sum of the intensities of light. Since Lambert–Beer’s law, extensively used in photometry, was not useful for explaining the decrease in the intensity of light by the biomass, an empirical expression was used. The measurement of the intensities from every direction was conducted in an externally illuminated cylindrical photo-bioreactor having an inner diameter of 60 mm and a working volume of 550 ml. The obtained results confirmed our estimation. The light distribution was applied to estimate the hydrogen production by R. capsulatus ST-410 using the same photo-bioreactor. The overall hydrogen-production rate was successfully estimated.

Cytochrome c' from Rhodobacter capsulatus confers increased resistance to nitric oxide.

We report the cloning and sequencing of the gene containing cytochrome c' (cycP) from the photosynthetic purple bacterium Rhodobacter capsulatus and the regions flanking that gene. Mutant strains unable to synthesize cytochrome c' had increased sensitivity to nitrosothiols and to nitric oxide (which binds to the heme moiety of cytochrome c').

Enhanced nitrogenase activity in strains of Rhodobacter capsulatus that overexpress the rnf genes.

In the photosynthetic bacterium Rhodobacter capsulatus, a putative membrane-bound complex encoded by the rnfABCDGEH operon is thought to be dedicated to electron transport to nitrogenase. In this study, the whole rnf operon was cloned under the control of the nifH promoter in plasmid pNR117 and expressed in several rnf mutants. Complementation analysis demonstrated that transconjugants which integrated plasmid pNR117 directed effective biosynthesis of a functionally competent complex in R. capsulatus. Moreover, it was found that strains carrying pNR117 displayed nitrogenase activities 50 to 100% higher than the wild-type level. The results of radioactive labeling experiments indicated that the intracellular content of nitrogenase polypeptides was marginally altered in strains containing pNR117, whereas the levels of the RnfB and RnfC proteins present in the membrane were four- and twofold, respectively, higher than the wild-type level. Hence, the enhancement of in vivo nitrogenase activity was correlated with a commensurate overproduction of the Rnf polypeptides. In vitro nitrogenase assays performed in the presence of an artificial electron donor indicated that the catalytic activity of the enzyme was not increased in strains overproducing the Rnf polypeptides. It is proposed that the supply of reductants through the Rnf complex might be rate limiting for nitrogenase activity in vivo. Immunoprecipitation experiments performed on solubilized membrane proteins revealed that RnfB and RnfC are associated with each other and with additional polypeptides which may be components of the membrane-bound complex.

Projection structure of a transcriptional regulator, HupR, determined by electron cryo-microscopy

Large, well-ordered two-dimensional crystals of the histidine-tagged-HupR protein, a transcriptional regulator from the photosynthetic bacterium Rhodobacter capsulatus, were obtained by specific interaction with a Ni 2+-chelated lipid monolayer. HupR is a response regulator of the NtrC subfamily; it activates the transcription of the structural genes hupSLC, of [NiFe]hydrogenase. A projection map of the full-length protein at 9 Å resolution was obtained by electron cryo-microscopy and image analysis of frozen-hydrated two-dimensional crystals. The crystals have a p6 plane group with unit cell dimensions of a = b = 111.6(±1.0) Å, γ = 120.4(±0.5)°. The structure of the N-terminal domain of NtrC, the family to which HupR belongs, had been determined previously by NMR. The atomic coordinates of the N-terminal domain of NtrC, were compared to the structure obtained by cryo-electron microscope techniques of the whole HupR. These results provide the first structure at medium resolution of a whole transcription factor, HupR from the NtrC family.

Purification and characterization of pyruvate oxidoreductase from the photosynthetic bacterium Rhodobacter capsulatus

Pyruvate:ferredoxin (flavodoxin) oxidoreductase (POR) was purified 3050-fold to apparent homogeneity from the photosynthetic bacterium Rhodobacter capsulatus using ion-exchange, Reactive Red, and gel filtration chromatography. The isolated enzyme was sensitive to dilution and oxygen (especially when in dilute solution). The molecular mass of the native enzyme was determined by high performance liquid chromatography gel filtration to be 270±20 kDa. Since a subunit molecular mass of 130±5 kDa was found by denaturing gel electrophoresis, POR from R. capsulatus thus appears to be a homodimer. Electron paramagnetic resonance analysis showed that a free radical was formed upon the addition of pyruvate. This POR is shown to be an indiscriminate electron donor causing the full reduction of R. capsulatus flavodoxin (Fld), R. capsulatus ferredoxin I (FdI), R. capsulatus ferredoxin II (FdII), as well as the major plant-type ferredoxin (FdI) from Anabaena variabilis. The purified enzyme can couple the oxidation of pyruvate to the reduction of nitrogenase in a coupled system with either R. capsulatus ferredoxins or nif-specific flavodoxin, NifF; (Fld>FdI>FdII). Immunoblot analysis shows that R. capsulatus POR is constitutively synthesized, with synthesis augmented under nitrogen-fixing conditions (34±13%) and decreased in acetate and aerobically grown cells.

The atpIBEXF operon coding for the F0 sector of the ATP synthase from the purple nonsulfur photosynthetic bacterium Rhodobacter capsulatus.

The atpIBEXF operon coding for the F0 sector of the ATP synthase from Rhodobacter capsulatus was cloned and sequenced. The genes for the five subunits were present in the order: atpI (subunit I), atpB (subunit a), atpE (subunit c), atpX (subunit b'), and atpF (subunit b). The transcription initiation site was defined by primer-extension analysis. A duplicated and divergent copy of the b subunit gene (subunit b') was present. This duplication is found only in photosynthetic prokaryotes and in plant chloroplasts. F0 deletion mutants formed tiny colonies during anaerobic growth in the dark but could not sustain continuous growth. Based on the results of the present work, we conclude that a functioning ATP synthase is essential for normal growth under all conditions tested.

Rhodobacter capsulatus HypF is involved in regulation of hydrogenase synthesis through the HupUV proteins.

The photosynthetic bacterium Rhodobacter capsulatus contains a membrane-bound [NiFe]hydrogenase encoded by the hupSL genes. We show in this study that hypF mutants are devoid of hydrogenase activity and lack the HupL protein. We also observed that, in contrast to the wild-type strain B10, transcription of the hupSL genes was not stimulated by H2 in the hypF mutants RS13 and BSE19. Complementation of the hypF mutants with the plasmid borne hypF gene restored hydrogenase activity to wild-type levels and inducibility by H2. The R. capsulatus hupU and hupV gene products share significant similarities with the small (HupS) and the large (HupL) hydrogenase subunits, respectively. Active HupUV proteins can catalyze the hydrogen-deuterium exchange reaction. In whole cells, this H-D exchange is distinguishable from the H-D exchange catalyzed by the membrane-bound HupSL proteins by its insensitivity to O2 and to acetylene. By measuring the formation of H2 and HD in exchange with D2 uptake, we demonstrated that the hypF mutants have no active HupUV nor HupSL proteins. H-D exchange activity, of both HupUV and HupSL, was restored by hypF gene complementation. These data indicate that the HypF protein participates not only in the maturation of HupSL, but also in the maturation of the HupUV proteins and that the latter are involved in the cellular response to H2.

Structural study of the response regulator HupR from Rhodobacter capsulatus. electron microscopy of two-dimensional crystals on a nickel-chelating lipid

Two-dimensional crystals of the histidine-tagged-HupR protein, a transcriptional regulator from the photosynthetic bacterium Rhodobacter capsulatus, were obtained upon specific interaction with a Ni 2+-chelated lipid monolayer. HupR is a response regulator of the NtrC family; it activates the transcription of the structural genes, hupSLC, of the [NiFe]hydrogenase. The lipid (Ni-NTA-DOGA) uses the metal chelator nitrilotriacetic group as the hydrophilic headgroup and contains unsaturated oleyl tails to provide the fluidity necessary for two-dimensional protein crystallization. A projection map of the full-length protein at 18 Å resolution was generated by analysing electron microscopy micrographs of negatively stained crystals. The HupR protein appeared to be dimeric and revealed a characteristic “propeller-like” motif. Each monomer forms an L-shaped structure.

The Rhodobacter capsulatus hupSLC promoter: identification of cis-regulatory elements and of trans-activating factors involved in H2 activation of hupSLC transcription.

The [NiFe]hydrogenase of the photosynthetic bacterium Rhodobacter capsulatus is encoded by the structural hupSLC operon, the expression of which is induced by H2. H2 activation was no longer observable in chromosomal hupR mutants, an indication that HupR is implicated directly in the activation by H2 of hupS gene expression. The transcriptional start site of the hupS promoter, determined by primer extension mapping, was located 55 nucleotides upstream from the translational start codon of the hupS gene. Regulatory sequences were identified by serial 5' deletions of the 300bp hupS promoter-regulatory region (phupS) and phupS-lacZ translational fusions. Cis-regulatory sequences capable of interacting with two transcription factors, IHF and HupR, a response regulator of the NtrC subfamily, were studied by electrophoretic mobility shift assays (EMSAs). The R. capsulatus IHF and HupR proteins were overexpressed in Escherichia coli and purified by affinity chromatography. IHF binds to a site, 5'-TCACACACCATTG, centred at -87 nt from the transcription start site. The HupR protein binds to one site within the -162 to -152 nt region, which contains the palindromic sequence 5'-TTG-R5-CAA. By the use of 5' deletions and site-directed mutagenesis of the -162/-152 region, this palindrome was shown to be required for in vivo hupS transcriptional activation by H2.

Differential levels of specific cytochrome c biogenesis proteins in response to oxygen: analysis of the ccl operon in Rhodobacter capsulatus.

The photosynthetic bacterium Rhodobacter capsulatus synthesizes c-type cytochromes under a variety of growth conditions. For example, under aerobic growth, c-type cytochromes are synthesized as part of an electron transport pathway, using oxygen as the terminal electron acceptor. Anaerobically in the light, R. capsulatus requires cytochrome bc1 and other c-type cytochromes for the photosynthetic electron transport pathway. It is shown here that the ccl1 and ccl2 genes of R. capsulatus are required for the synthesis of all c-type cytochromes, including the cytochrome c' protein of unknown function but of structural similarity to cytochrome b562. Polar and nonpolar mutations constructed in each gene demonstrated that the ccl12 genes form an operon. Expression of the ccl12 genes was examined by using lacZ and phoA fusions as translational reporters. Primer extension analysis was used to determine transcriptional control and the start site of the ccl12 promoter. Finally, antiserum to the Ccl2 protein was used to quantitate levels of Ccl2 under six different growth conditions. The Ccl2 protein is present at 20-fold-higher levels under conditions where oxygen is present. In contrast, other cytochromes c biogenesis proteins, HelA and HelX, previously shown to be part of an helABCDX operon, are at relatively similar levels under these six growth conditions. This discovery is discussed in terms of the physiology and evolution of cytochromes c biogenesis, with particular attention to oxidative environments.

Positive selection systems for discovery of novel polyester biosynthesis genes based on fatty acid detoxification.

The photosynthetic bacterium Rhodobacter capsulatus can grow with short- to long-chain fatty acids as the sole carbon source (R. G. Kranz, K. K. Gabbert, T. A. Locke, and M. T. Madigan, Appl. Environ. Microbiol. 63:3003-3009, 1997). Concomitant with growth on fatty acids is the production to high levels of the polyester storage compounds called polyhydroxyalkanoates (PHAs). Here, we describe colony screening and selection systems to analyze the production of PHAs in R. capsulatus. A screen with Nile red dissolved in acetone distinguishes between PHA producers and nonproducers. Unlike the wild type, an R. capsulatus PhaC- strain with the gene encoding PHA synthase deleted is unable to grow on solid media containing high concentrations of certain fatty acids. It is proposed that this deficiency is due to the inability of the PhaC- strain to detoxify the surrounding medium by consumption of fatty acids and their incorporation into PHAs. This fatty acid toxicity phenotype is used in selection for the cloning and characterization of heterologous phaC genes.

Polyhydroxyalkanoate production in Rhodobacter capsulatus: genes, mutants, expression, and physiology.

Like many other prokaryotes, the photosynthetic bacterium Rhodobacter capsulatus produces high levels of polyhydroxyalkanoates (PHAs) when a suitable carbon source is available. The three genes that are traditionally considered to be necessary in the PHA biosynthetic pathway, phaA (beta-ketothiolase), phaB (acetoacetylcoenzyme A reductase), and phaC (PHA synthase), were cloned from Rhodobacter capsulatus. In R. capsulatus, the phaAB genes are not linked to the phaC gene. Translational beta-galactosidase fusions to phaA and phaC were constructed and recombined into the chromosome. Both phaC and phaA were constitutively expressed regardless of whether PHA production was induced, suggesting that control is posttranslational at the enzymatic level. Consistent with this conclusion, it was shown that the R. capsulatus transcriptional nitrogen-sensing circuits were not involved in PHA synthesis. The doubling times of R. capsulatus transcriptional nitrogen-sensing circuits were not involved in PHA synthesis. The doubling times of R. capsulatus grown on numerous carbon sources were determined, indicating that this bacterium grows on C2 to C12 fatty acids. Grown on acetone, caproate, or heptanoate, wild-type R. capsulatus produced high levels of PHAs. Although a phaC deletion strain was unable to synthesize PHAs on any carbon source, phaA and phaAB deletion strains were able to produce PHAs, indicating that alternative routes for the synthesis of substrates for the synthase are present. The nutritional versatility and bioenergetic versatility of R. capsulatus, coupled with its ability to produce large amounts of PHAs and its genetic tractability, make it an attractive model for the study of PHA production.

A [2Fe-2S] ferredoxin (FdVI) is essential for growth of the photosynthetic bacterium Rhodobacter capsulatus.

The physiological function of Rhodobacter capsulatus FdVI, a [2Fe-2S] ferredoxin, was investigated by the cloning, sequence analysis, and mutagenesis of its structural gene, called fdxE. The DNA region surrounding fdxE was mapped, and the nucleotide sequence of a 4.2-kb fragment was determined. fdxE is preceded by a sequence that is very similar to a sigma54 recognition site and is followed by a putative transcription stop signal, suggesting that fdxE forms a separate cistron. Two open reading frames were identified upstream and downstream of fdxE and were named ORFE0 and ORFE1, respectively. The former may encode a polypeptide having 34% similarity with HtrA, a serine protease found in enteric bacteria. ORFE1 is homologous to purU, a gene involved in purine biosynthesis. Interposon mutagenesis of fdxE was unsuccessful when attempted on the wild-type strain B10. Disruption of fdxE could be achieved only in strains harboring an additional copy of fdxE on a plasmid. Mutants obtained in this way and carrying a plasmid-borne copy of fdxE under the control of the nifH promoter grew only in N-free medium, thus demonstrating that fdxE expression is required for growth. Nevertheless, such mutants were found to spontaneously revert at a frequency of 5 x 10(-6) to an apparent wild-type phenotype, although they contained no detectable amount of FdVI. Taken together, the results indicate that FdVI is required for an essential metabolic function in R. capsulatus and that this FdVI dependence could be relieved by a single-mutation event. In accordance, FdVI biosynthesis was found to be constitutive in R. capsulatus.

PH-Metric Study of Reaction Centers from Photosynthetic Bacteria in Micellular Solutions: Protonatable Groups Equilibrate with the Aqueous Bulk Phase

Hydrogen ion equilibria of the reaction center protein from photosynthetic purple bacteria Rhodobacter sphaeroides and Rhodobacter capsulatus dissolved in micellular solution were studied by acid-base titration to estimate the water accessibility of protonatable residues of the protein determined from structural data. The ionizable amino acids of the reaction center underwent protonation-deprotonation with protons from the interfacial layer, which, however, exchanged protons from the aqueous bulk phase. The equilibrium was described in terms of the buffering capacity of the multiphase system. The detergents decreased the proton activity coefficient (increased the buffering capacity) of the aqueous solution by a factor of 0.33 (in 0.03% Triton X-100 and LDAO) and 0.12 (0.04% dodecyl beta-D-maltoside). The observed buffering capacities of the reaction center protein were large and detergent-dependent. However, corrections for proton activities made the pH dependence of buffering capacities in different detergents uniform and similar to that expected from the number and pK values of protonatable groups of the protein. The vast majority of protonatable amino acids of the reaction center are in protonation equilibria with the aqueous bulk phase on an extended time scale.

Assay and properties of acetone carboxylase, a novel enzyme involved in acetone-dependent growth and CO2 fixation in Rhodobacter capsulatus and other photosynthetic and denitrifying bacteria.

The photosynthetic bacterium Rhodobacter capsulatus is able to grow, in the presence of carbon dioxide, under anaerobic (photosynthetic) conditions with the solvents acetone or butanone as carbon source. The carboxylation of acetone to form acetoacetate is the most likely initial step in acetone metabolism. This paper describes an assay for acetone carboxylation, in which fixation of radiolabeled carbon dioxide from NaH14CO3 is measured in the presence of acetone, ATP, magnesium ions and acetyl-CoA. Acetone carboxylase activity was specifically induced by growth of R. capsulatus on acetone or butanone and was associated with a high-molecular-mass protein complex containing two major polypeptides, of 70 and 85 kDa. Partial purification of the activity was achieved by FPLC ion-exchange chromatography, which confirmed that the 70 and 85 kDa proteins were subunits of the enzyme and suggested that at least one additional protein (60 kDa) may be associated with carboxylase activity. N-terminal sequences of the two major subunits were not significantly similar to any other carboxylases in the databases and neither contained covalently bound biotin, indicating that the enzyme represents a novel type of carboxylase. Acetone carboxylase activity was also demonstrated in cell-free extracts of acetone-grown Rhodomicrobium vannielii and the denitrifying bacterium Thiosphaera pantotropha.

Regulation of synthesis of pyruvate carboxylase in the photosynthetic bacterium Rhodobacter capsulatus.

The synthesis of pyruvate carboxylase (PC) was studied by using quantitative immunoblot analysis with an antibody raised against PC purified from Rhodobacter capsulatus and was found to vary 20-fold depending on the growth conditions. The PC content was high in cells grown on pyruvate or on carbon substrates metabolized via pyruvate (lactate, D-malate, glucose, or fructose) and low in cells grown on tricarboxylic acid (TCA) cycle intermediates or substrates metabolized without intermediate formation of pyruvate (acetate or glutamate). Under dark aerobic growth conditions with lactate as a carbon source, the PC content was approximately twofold higher than that found under light anaerobic growth conditions. The results of incubation experiments demonstrate that PC synthesis is induced by pyruvate and repressed by TCA cycle intermediates, with negative control dominating over positive control. The content of PC in R. capsulatus cells was also directly related to the growth rate in continuous cultures. The analysis of intracellular levels of pyruvate and TCA cycle intermediates in cells grown under different conditions demonstrated that the content of PC is directly proportional to the ratio between pyruvate and C4 dicarboxylates. These results suggest that the regulation of PC synthesis by oxygen and its direct correlation with growth rate may reflect effects on the balance of intracellular pyruvate and C4 dicarboxylates. Thus, this important enzyme is potentially regulated both allosterically and at the level of synthesis.

Programmed Cell Death in Prokaryotes

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 swarmer cell formation and differentiation in Caulobacter cereus, sporu-lation 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 × 106 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.