Immunochemical localization of NADP-specific isocitrate dehydrogenase in Escherichia coli

Formation of a fibrous structure on the surface of Legionella pneumophila associated with exposure of DotH and DotO proteins after intracellular growth

Legionella pneumophila grows in human alveolar macrophages and resides within a phagosome that initially lacks proteins associated with the endocytic pathway. Required for targeting to this unique location is the Dot/Icm complex, which is highly similar to conjugative DNA transfer apparatuses. Here, we show that exposure to three distinct inducing conditions resulted in the formation of a fibrous structure on the bacterial cell surface that contained the DotH and DotO proteins. These conditions included: (i) incubation for 2 h with mouse bone marrow-derived macrophages; (ii) incubation for 2 h in macrophage-conditioned media; or (iii) replication of bacteria for 22 h within macrophages. Introduction of bacteria harbouring the surface-exposed DotH and DotO onto a fresh monolayer resulted in loss of the surface localization of DotH and DotO shortly after uptake. Treatments that resulted in the production of the fibrous structure enhanced the rate at which the bacteria were internalized, but there was no corresponding increase in the efficiency of intracellular growth compared with bacteria that had been cultured in broth using conditions that resulted in maximal intracellular growth. These data indicate that the surface-exposed DotH and DotO on L. pneumophila may act either just before lysis from the macrophage or at the earliest stages of infection, transiently relocating in a fibrous structure on the bacterial cell surface.

Regulation of acetate metabolism by protein phosphorylation in enteric bacteria

Growth of enteric bacteria on acetate as the sole source of carbon and energy requires operation of a particular anaplerotic pathway known as the glyoxylate bypass. In this pathway, two specific enzymes, isocitrate lyase and malate synthase, are activated to divert isocitrate from the tricarboxylic acid cycle and prevent the quantitative loss of acetate carbons as carbon dioxide. Bacteria are thus supplied with the metabolic intermediates they need for synthesizing their cellular components. The channeling of isocitrate through the glyoxylate bypass is regulated via the phosphorylation/dephosphorylation of isocitrate dehydrogenase, the enzyme of the tricarboxylic acid cycle which competes for a common substrate with isocitrate lyase. When bacteria are grown on acetate, isocitrate dehydrogenase is phosphorylated and, concomitantly, its activity declines drastically. Conversely, when cells are cultured on a preferred carbon source, such as glucose, the enzyme is dephosphorylated and recovers full activity. Such reversible phosphorylation is mediated by an unusual bifunctional enzyme, isocitrate dehydrogenase kinase/phosphatase, which contains both modifying and demodifying activities on the same polypeptide. The genes coding for malate synthase, isocitrate lyase, and isocitrate dehydrogenase kinase/phosphatase are located in the same operon. Their expression is controlled by a complex dual mechanism that involves several transcriptional repressors and activators. Recent developments have brought new insights into the nature and mode of action of these different regulators. Also, significant advances have been made lately in our understanding of the control of enzyme activity by reversible phosphorylation. In general, analyzing the physiological behavior of bacteria on acetate provides a valuable approach for deciphering at the molecular level the mechanisms of cell adaptation to the environment.

Immunocytochemical ultrastructural analysis of chromatophore membrane formation in Rhodospirillum rubrum

An immunocytochemical ultrastructural study of Rhodospirillum rubrum cultured under semiaerobic conditions was conducted to correlate the localization of functional components with membrane formation. R. rubrum is a facultatively phototrophic organism. Under reduced oxygen, this bacterium forms an intracytoplasmic chromatophore membrane that is the site of the photosynthetic apparatus. Immunogold techniques were used to localize intracellular protein antigens associated with the photosynthetic apparatus. Antibody, demonstrated by immunoblotting to be specific for the reaction center and light-harvesting photochemical components, was conjugated to colloidal gold particles and used for direct immunolabeling of fixed, sectioned specimens. Membrane invaginations appeared by 4 h after transition to induction conditions, and mature chromatophore membrane was abundant by 22 h. The occurrence of chromatophore membrane was correlated with bacteriochlorophyll a content and the density of the immunolabel. In uninduced (aerobic) cells and those obtained from cultures 0.5 h posttransition, the immunogold preferentially labeled the peripheral area of the cell. In contrast, in cells obtained after 22 h of induction, the central region of the cell was preferentially immunolabeled. These findings provided immunocytochemical evidence supporting the hypothesis that the chromatophore membrane is formed by invagination of the cytoplasmic membrane.

Localization of the enantiozymes of 6-hydroxy-nicotine oxidase in Arthrobacter oxidans by electron immunochemistry

During the course of growth of Arthrobacter oxidans, induction of the enantiozymes 6-hydroxy-D-nicotine oxidase and 6-hydroxy-L-nicotine oxidase occurred in the presence of DL-nicotine. Cryoultramicrotomed sections obtained from cells grown to stationary phase were gold immunolabeled. The results obtained demonstrate that both enzymes are localized in the cytoplasm.

Homogeneity among mitochondria revealed by a constant proportion of their enzymes

The homogeneity or heterogeneity at the enzyme level of mitochondria has not been directly demonstrated and is important for many studies. To clarify this point, carbamoyl phosphate synthase (ammonia), glutamate dehydrogenase and mitochondrial adenosine triphosphatase (F1) were located in rat liver by immunolabeling using protein A-gold. Measurements of the number of gold particles per square micron of cross sectional images of mitochondria permit to assess the relative molecular concentration of the three enzymes and, most interestingly, it presents the first evidence that different mitochondria in rat liver cells have the same relative proportion of the three enzymes. Since they have vastly different half-lives, bulk or unregulated autophagy as the main mechanism regulating the turnover of these enzymes seems unlikely.