Lytic Cycle of Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular pathogen within the phylum Apicomplexa. This protozoan parasite is one of the most widespread, with a broad host range including many birds and mammals and a geographic range that is nearly worldwide. While infection of healthy adults is usually relatively mild, serious disease can result in utero or when the host is immunocompromised. This sophisticated eukaryote has many specialized features that make it well suited to its intracellular lifestyle. In this review, we describe the current knowledge of how the asexual tachyzoite stage of Toxoplasma attaches to, invades, replicates in, and exits the host cell. Since this process is closely analogous to the way in which viruses reproduce, we refer to it as the Toxoplasma “lytic cycle.”     

Occurrence of Cell Lytic Enzymes in Blue-Green Bacteria

Detergent extracts of three blue-green bacteria (Agmenellum quadruplicatum strain BG1, Anacystis nidulans strain TX20, and Nostoc sp. strain MAC) contained enzymes capable of lysing suspensions of Micrococcus lysodeikticus. The enzyme preparation from A. quadruplicatum released soluble reducing fragments from purified peptidoglycan. The lytic activity exhibited a pH optimum between 6 and 7, was relatively heat stable, and was susceptible to attack by proteolytic enzymes. These results extend the range of bacterial types exhibiting cell lytic activity as well as confirm the existence of the lytic system commonly observed in "water blooms".     

Atomic Force Microscopy Analysis of the Acinetobacter baumannii Bacteriophage AP22 Lytic Cycle

Background

Acinetobacter baumannii is known for its ability to develop resistance to the major groups of antibiotics, form biofilms, and survive for long periods in hospital environments. The prevalence of infections caused by multidrug-resistant A. baumannii is a significant problem for the modern health care system, and application of lytic bacteriophages for controlling this pathogen may become a solution.

Methodology/Principal Findings

In this study, using atomic force microscopy (AFM) and microbiological assessment we have investigated A. baumannii bacteriophage AP22, which has been recently described. AFM has revealed the morphology of bacteriophage AP22, adsorbed on the surfaces of mica, graphite and host bacterial cells. Besides, morphological changes of bacteriophage AP22-infected A. baumannii cells were characterized at different stages of the lytic cycle, from phage adsorption to the cell lysis. The phage latent period, estimated from AFM was in good agreement with that obtained by microbiological methods (40 min). Bacteriophage AP22, whose head diameter is 62±1 nm and tail length is 88±9 nm, was shown to disperse A. baumannii aggregates and adsorb to the bacterial surface right from the first minute of their mutual incubation at 37°C.

Conclusions/Significance

High rate of bacteriophage AP22 specific adsorption and its ability to disperse bacterial aggregates make this phage very promising for biomedical antimicrobial applications. Complementing microbiological results with AFM data, we demonstrate an effective approach, which allows not only comparing independently obtained characteristics of the lytic cycle but also visualizing the infection process.     

Cell Cycle Regulation

Drosophila researchers met in sunny San Diego for the 49th annual meeting of The Genetics Society of America. It was cold outside and even colder inside. Like last year, ‘Mitosis, Meiosis and Cell Division’ was no longer a session. Instead, we searched out and covered talks and posters in ‘Cell Division and Growth Control’, ‘Gametogenesis’, ‘Cytoskeleton and Cell Biology’ and ‘Genome and Chromosome Structure’. We split up for maximal coverage and re-grouped later for the Workshop on Cell Cycle and Checkpoints. We apologize in advance for the brevity or omission of some reports.     

Chromatin-Like Structures in Polyoma Virus and Simian Virus 40 Lytic Cycle

Nucleoprotein complexes containing viral DNA and cellular histones were extracted from nuclei of permissive cells infected with polyoma virus or simian virus 40 (SV40) and examined by electron microscopy. Polyoma and SV40 nucleoprotein complexes are almost identical. They appear as relaxed circular molecules consisting of 20 to 21 globular particles interconnected by thin filaments. Their contour length in 0.02 M salt is 2.7 times shorter than that of viral DNA form I obtained after dissociation of the proteins in 1 M NaCl. The nucleosomes have an average diameter of 12.5 nm. Each nucleosome contains 175 to 205 DNA base pairs condensed fivefold in length. The nucleosomes are regularly spaced on the circular molecule. The internucleosomal filaments are made of naked DNA, and each filament contains about 55 base pairs. The partial sensitivity of the nucleoprotein complex to cleavage by EcoR1 endonuclease suggests that the nucleosomes are not formed at specific sites on the viral genome. Faster sedimenting nucleoprotein complexes containing replicative intermediates were studied. Isopycnic centrifugation in metrizamide gradients in the absence of aldehyde fixation showed that these molecules conserved the same DNA-to-protein ratio as the form I DNA-containing complexes.     

细胞周期检控点

细胞周期是高度有组织的时序调控过程,受到DNA损伤检控点、DNA复制检控点和纺锤体检控点等细胞周期检控点的精确调控。细胞周期检控点的作用主要是调节细胞周期的时序转换,以确保DNA复制、染色体分离等细胞重要生命活动的高度精确性,并对DNA损伤、DNA复制受阻、纺锤体组装和染色体分离异常等细胞损伤及时做出反应,以防止突变和遗传不稳定的发生。细胞周期检控点的功能缺陷,将导致细胞基因组的不稳定,与细胞癌变密切相关。因此细胞周期检控点对于维持细胞遗传信息的稳定性和完整性以及防止细胞癌变和遗传疾病的发生起着至关重要的作用。     

Introducing Cell Cycle

No Abstract Available     

The Cell Cycle

I review recent advances in our knowledge of the eucaryoticcell cycle: the set of processes by which cells grow and divide.Genetic approaches to the cell cycle of somatic cells identifieda pathway of events where the initiation of each event was dependenton the successful completion of the preceding event, as wellas a single key gene, cdc2, that is required both at the beginningand at the end of the cell cycle. The alternative approach ofstudying the cell cycle biochemically in early embryos providedevidence for a cytoplasmic oscillator which alternated betweenmitosis-inducing and interphase-inducing states and identifiedthe mitosis-inducing component as maturation promoting factor(MPF). These two very different views of the cell cycle initiallyseemed irreconcilable. However, a link between the somatic andembryonic cell cycles was provided by the recent discovery thatthe cdc2 protein is one of the components of MPF. In the embryoniccell cycle the activation of MPF and induction of mitosis istriggered by the accumulation of a protein named cyclin whichbecomes a component of MPF. Somehow, MPF induces the proteolyticdegradation of cyclin, which inturn allows MPF to be inactivatedand allows the cell cycle to pass from mitosis into interphase.The more complex cell cycle of somatic cells is probably derivedfrom the embryonic cyclin-based oscillator by imposing a systemof checks and balances on the accumulation and destruction ofcyclin. I also present some thoughts on the relationships between scienceand society, and comment on the way in which scientists describetheir work to the lay world.     

Investigation of the Structure of Rhizopus Cell Wall with Lytic Enzymes

The components and structure of the cell wall of Rhizopus delemar were investigated using purified lytic enzymes, protease and chitosanase from Bacillus R-4 and chitinase II from Streptomyces orientalis. When these enzymes were used individually they only partially lysed the cell wall, but when allowed to react on the cell wall together, a complete lysis was achieved by cooperative action. These modes of action on the cell wall and the chemical and morphological data suggested that the cell wall structure was different in Rhizopus delemar of Zygomycetes from filamentous fungi of Euascomycetes and that its wall structure might be composed mainly of chitin fibers cemented by chitosan and protein or peptides scattered in a mosaic manner.     

Distribution of Yeast Cell Wall Lytic Activity among Microorganisms

An α-glucosidase has been isolated from the mycelia of Penicillium purpurogenum in electrophoretically homogeneous form, and its properties have been investigated. The enzyme had a molecular weight of 120,000 and an isoelectric point of pH 3.2. The enzyme had a pH optimum at 3.0 to 5.0 with maltose as substrate. The enzyme hydrolyzed not only maltose but also amylose, amylopectin, glycogen, and soluble starch, and glucose was the sole product from these substrates. The Km value for maltose was 6.94×10^?4 m. The enzyme hydrolyzed phenyl α-maltoside to glucose and phenyl α-glucoside. The enzyme had α-glucosyltransferase activity, the main transfer product from maltose being maltotriose. The enzyme also catalyzed the transfer of α-glucosyl residue from maltose to riboflavin.     

Simian Virus 40-Host Cell Interaction During Lytic Infection

Both exponentially growing and serum-arrested subcloned CV-1 cell cultures were infected with simian virus 40 (SV40). By 24 h after infection 96% of the nuclei of these permissive cells contained SV40 T-antigen. Analysis of the average DNA content per cell at various times after infection indicated that by 24 h most of the cells contained amounts of DNA similar to those normally found in G(2) cells. Analysis of cell cycle distributions indicated that a G(2) DNA complement was maintained by over 90% of the cells in the infected populations 24 to 48 h postinfection. Cells continued to synthesize SV40 DNA during the first 50 h after infection, and cytopathic effect was first observed 60 h after inoculation. After infection the number of mitotic cells that could be recovered by selective detachment decreased precipitously and was drastically reduced by 24 h. A study of the kinetics of decline in the number of mitotic cells suggests that this decline is related to an event during the cell cycle at or near the G(1)-S-phase border upon which commencement of SV40 DNA replication apparently depends. It was concluded that after SV40 infection, stationary cells are induced to cycle, and cycling cells complete one round of cellular DNA synthesis but do not divide. Although the infected cells continue to synthesize viral DNA, they do not appear able to reinitiate cellular DNA replication units. These results imply that the abundance of T-antigen (produced independently of cell cycle phase) in the presence of the enzymes required for continued DNA synthesis is not sufficient for reinitiation of cellular DNA synthesis.     

Red Yeast Cell Lysis by Red Yeast Cell Wall Lytic Enzyme and Protease

The purified red yeast cell wall lytic enzyme of Penicillium lilacinum No. 2093 has a potent saccharifying activity against cell walls, but the living cell lytic activity of it is considerably lower than that of the culture filtrate. Therefore, the living cell lytic factors in the culture filtrate were examined. The alkaline protease of Pen. lilacinum played an important role for living cell lysis. The synergistic effect on living cell lysis was also detected, when acid proteases from various origins were combined with the cell wall lytic enzyme. These results indicated that the protein layers of red yeast cell surface inhibited the action of a glycanase,cell wall lytic enzyme, and the protein molecule contributed to retain the rigid structure of the wall.