Functional inclusions in prokaryotic cells

Recent approaches for enhanced production of microbial polyhydroxybutyrate: Preparation of biocomposites and applications

In modern decades, an increase in environmental awareness has attracted the keen interest of researchers to investigate eco-sustainable, recyclable materials to minimize reliance on petroleum-based polymeric compounds. Poly (3-hydroxybutyrate) is amorphous, linear, and biodegradable bacterial polyesters that belong to the polyhydroxyalkanoates family with enormous applications in many fields. The present review provides comprehensive information on polyhydroxybutyrate production from different biomass feedstock. Various studies on PHB production by genetically engineered bacterial cells and optimization of parameters have been discussed. Recent technological innovation in processing polyhydroxybutyrate-based biocomposite through the different process has also been examined. Besides this, the potential applications of the derived competent biocomposites in the other fields have been depicted.

New insights into roles of acidocalcisomes and contractile vacuole complex in osmoregulation in protists

While free-living protists are usually subjected to hyposmotic environments, parasitic protists are also in contact with hyperosmotic habitats. Recent work in one of these parasites, Trypanosoma cruzi, has revealed that its contractile vacuole complex, which usually collects and expels excess water as a mechanism of regulatory volume decrease after hyposmotic stress, has also a role in cell shrinking when the cells are submitted to hyperosmotic stress. Trypanosomes also have an acidic calcium store rich in polyphosphate (polyP), named the acidocalcisome, which is involved in their response to osmotic stress. Here, we review newly emerging insights on the role of acidocalcisomes and the contractile vacuole complex in the cellular response to hyposmotic and hyperosmotic stresses. We also review the current state of knowledge on the composition of these organelles and their other roles in calcium homeostasis and protein trafficking.

Acidocalcisomes

Acidocalcisomes are acidic organelles containing calcium and a high concentration of phosphorus in the form of pyrophosphate (PP(i)) and polyphosphate (poly P). Organelles with these characteristics have been found from bacteria to human cells implying an early appearance and persistence over evolutionary time or their appearance by convergent evolution. Acidification of the organelles is driven by the presence of vacuolar proton pumps, one of which, the vacuolar proton pyrophosphatase, is absent in animals, where it is substituted by a vacuolar proton ATPase. A number of other pumps, antiporters, and channels have been described in acidocalcisomes of different species and are responsible for their internal content. Enzymes involved in the synthesis and degradation of PP(i) and poly P are present within the organelle. Acidocalcisomes function as storage sites for cations and phosphorus, and participate in PP(i) and poly P metabolism, calcium homeostasis, maintenance of intracellular pH, and osmoregulation. Experiments in which the acidocalcisome Ca(2+)-ATPase of different parasites were downregulated or eliminated, or acidocalcisome Ca(2+) was depleted revealed the importance of this store in Ca(2+) signaling needed for host invasion and virulence. Acidocalcisomes interact with other organelles in a number of organisms suggesting their association with the endosomal/lysosomal pathway, and are considered part of the lysosome-related group of organelles.

Evolution of acidocalcisomes and their role in polyphosphate storage and osmoregulation in eukaryotic microbes

Acidocalcisomes are acidic electron-dense organelles, rich in polyphosphate (poly P) complexed with calcium and other cations. While its matrix contains enzymes related to poly P metabolism, the membrane of the acidocalcisomes has a number of pumps (Ca(2+)-ATPase, V-H(+)-ATPase, H(+)-PPase), exchangers (Na(+)/H(+), Ca(2+)/H(+)), and at least one channel (aquaporin). Acidocalcisomes are present in both prokaryotes and eukaryotes and are an important storage of cations and phosphorus. They also play an important role in osmoregulation and interact with the contractile vacuole complex in a number of eukaryotic microbes. Acidocalcisomes resemble lysosome-related organelles (LRO) from mammalian cells in many of their properties. They share similar morphological characteristics, acidic properties, phosphorus contents and a system for targeting of their membrane proteins through adaptor complex-3 (AP-3). Storage of phosphate and cations may represent the ancestral physiological function of acidocalcisomes, with cation and pH homeostasis and osmoregulatory functions derived following the divergence of prokaryotes and eukaryotes.

Structure and mechanisms of a protein-based organelle in Escherichia coli

Many bacterial cells contain proteinaceous microcompartments that act as simple organelles by sequestering specific metabolic processes involving volatile or toxic metabolites. Here we report the three-dimensional (3D) crystal structures, with resolutions between 1.65 and 2.5 angstroms, of the four homologous proteins (EutS, EutL, EutK, and EutM) that are thought to be the major shell constituents of a functionally complex ethanolamine utilization (Eut) microcompartment. The Eut microcompartment is used to sequester the metabolism of ethanolamine in bacteria such as Escherichia coli and Salmonella enterica. The four Eut shell proteins share an overall similar 3D fold, but they have distinguishing structural features that help explain the specific roles they play in the microcompartment. For example, EutL undergoes a conformational change that is probably involved in gating molecular transport through shell protein pores, whereas structural evidence suggests that EutK might bind a nucleic acid component. Together these structures give mechanistic insight into bacterial microcompartments.

Protein-based organelles in bacteria: carboxysomes and related microcompartments

Many bacteria contain intracellular microcompartments with outer shells that are composed of thousands of protein subunits and interiors that are filled with functionally related enzymes. These microcompartments serve as organelles by sequestering specific metabolic pathways in bacterial cells. The carboxysome, a prototypical bacterial microcompartment that is found in cyanobacteria and some chemoautotrophs, encapsulates ribulose-l,5-bisphosphate carboxylase/oxygenase (RuBisCO) and carbonic anhydrase, and thereby enhances carbon fixation by elevating the levels of CO2 in the vicinity of RuBisCO. Evolutionarily related, but functionally distinct, microcompartments are present in diverse bacteria. Although bacterial microcompartments were first observed more than 40 years ago, a detailed understanding of how they function is only now beginning to emerge.

Alteration of cylindrospermopsin production in sulfate- or phosphate-starved cyanobacteriumAphanizomenon ovalisporum

The effect of sulfate and phosphate deprivation on cell growth and cylindrospermopsin level was studied in Aphanizomenon ovalisporum ILC-164. Sulfate starvation induced a characteristic reduction of cylindrospermopsin pool size on the basis of cell number and unit of dry mass of culture. Phosphorous starvation of A. ovalisporum cultures induced a lesser reduction of cylindrospermopsin pool size. This divergence in the pool size of cylindrospermopsin may be the consequence of different growth rate. To show the metabolic changes concomitant with reduction of cylindrospermopsin pool size were obtained by measurement of ATP sulfurylase and alkaline phosphatase activity. The present study is the first concerning the cylindrospermopsin content under sulfate starvation and discusses it in relation to phosphorous starvation.

Microbial inclusions with special reference to PHA inclusions and intracellular boundary envelopes

Polyhydroxyalkanoates (PHAs), in the form of metabolic storage reserves, are assembled in intracellular cytoplasmic inclusions, often called granules. This review discusses both the structure and function of this assembly. In addition an overview of other microbial cellular inclusions is presented. This is not a compilation of all such structures but a description of those that are similar in many ways to either the structure or function of the PHA inclusions and are made up of monolayer envelopes and their storage compounds. Not unique, such inclusions provide many similar examples which, in turn, provide useful analogies to the PHA inclusions. A study of the PHA inclusions has been carried out in a comparative electron microscope examination and by protein analysis of a number of organisms and E. coli transformants.

Something from almost nothing: carbon dioxide fixation in chemoautotrophs

The last decade has seen significant advances in our understanding of the physiology, ecology, and molecular biology of chemoautotrophic bacteria. Many ecosystems are dependent on CO2 fixation by either free-living or symbiotic chemoautotrophs. CO2 fixation in the chemoautotroph occurs via the Calvin-Benson-Bassham cycle. The cycle is characterized by three unique enzymatic activities: ribulose bisphosphate carboxylase/oxygenase, phosphoribulokinase, and sedoheptulose bisphosphatase. Ribulose bisphosphate carboxylase/oxygenase is commonly found in the cytoplasm, but a number of bacteria package much of the enzyme into polyhedral organelles, the carboxysomes. The carboxysome genes are located adjacent to cbb genes, which are often, but not always, clustered in large operons. The availability of carbon and reduced substrates control the expression of cbb genes in concert with the LysR-type transcriptional regulator, CbbR. Additional regulatory proteins may also be involved. All of these, as well as related topics, are discussed in detail in this review.

Isolation and characterization of a carboxysome shell gene from Thiobacillus neapolitanus

The gene coding for the major carboxysome shell peptide (csoS1) from Thiobacillus neapolitanus has been isolated and sequenced. Oligonucleotide primers for polymerase chain reaction (PCR) amplification of the 5' end of the gene were made possible by amino acid sequencing of the N-terminal residues of the shell peptide. A 41 bp PCR product was used as a probe to isolate the gene. The deduced amino acid composition of the 216 bp gene shows a high degree of hydrophobicity. The gene is located within a series of three repeated regions of DNA and appears to have arisen via gene duplication. The transcript of csoS1 is approximately 400 bases in length. The shell peptide shares significant homology with Synechococcus open reading frames implicated in carboxysome structure/assembly. These open reading frames and csoS1 are related and are probably members of a carboxysome gene family.

In situ assay of ribulose-1,5-bisphosphate carboxylase/oxygenase in Thiobacillus neapolitanus

Cells permeabilized with chloroform yielded ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) activities nearly equal to those of cell extracts, thus indicating that both cytoplasmic and carboxysomal RuBisCO are functional in situ. The carboxysomal and cytoplasmic RuBisCO both form the CO2-Mg2(+)-enzyme ternary complex, as evidenced by stabilization with 2-C-carboxy-D-arabinitol-1,5-bisphosphate (CABP), a potent competitive inhibitor of RuBisCO. The data are consistent with the hypothesis that the carboxysome is functional in carbon dioxide fixation.