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."     

Lytic action of cloned phi X174 gene E.

The phi X174 lysis gene E was placed under control of the lac promoter by cloning into the multicopy plasmid pBH20. Other phi X174 gene sequences were removed by nuclease digestion. Expression of gene E was shown to be necessary and sufficient to produce lysis phenomena exhibited by infection with intact phage. Lysis, its inhibition by MgSO4 and spermine, its progression through a spheroplasting stage, and its dependence on an early chloramphenicol-sensitive step were reproduced in clones induced for expression of the E gene product. Escherichia coli clones carrying the E gene not under lac control, and clones under lac control but only minimally induced for gene E expression, exhibited morphological aberrations consistent with the view that the mechanism by which gene E mediates cell lysis is related to host cell division processes.     

Lytic bacteriophages ofStreptococcus mutans

Three phages ofStreptococcus mutans were obtained and partially characterized. The three phages, designated M102, e10, and f1, were found to be strictly lytic, with host ranges restricted to only serotype c, e, and f strains of this species, respectively. Phage sensitivity was not correlated with the presence of plasmids, at least in host strains of serotypes c and e. Each phage produced clear plaques in a number of standard media, even in the presence of sucrose, indicating that the extracellular glucan polysaccharides (mutan) produced by the hosts from this substrate do not prevent phage adsorption and growth. The phages were similar in size and morphology, having icosahedral heads and long (283–287 nm), flexible, noncontractile tails. The genome of each phage was found to consist of linear, double-stranded DNA, 31–35 kb in length, with a base composition of 37–38% G+C. Restricting phage DNAs with four enzymes produced fragment patterns unique to each phage, but common bands between M102 and e10 and between e10 and f1 were produced byBamHI. Labeled e10 and M102 DNAs hybridized strongly with all three phage DNAs, indicating that they share some common sequences. The three phages appear to be more similar than expected and probably evolved from a common ancestor.     

Lytic activity of Pseudomonas bacteriophages

None of the 24 Pseudomonas syringae bacteriophages were found to be identical in the spectrum of lytic action. The phages were subdivided into five groups according to the number of sensitive bacterial strains and their qualitative composition.     

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.”     

Extracellular Lytic Enzymes of Myxococcus virescens

The aim of the investigation was to obtain large amounts of the bacteriolytic enzymes of Myxococcus virescens and to separate these enzymes from the non-bactcriolytic protemases produced by this organism. The bacteria were grown in Casitone broth. When the bacteriolytic activity had reached its maximal value, the cells were removed from the culture medium by centrifugation. Polyethylene glycol 4000 (20 g/1) and potassium phosphate (about 210 g/1) were added to the cell-free solution. The additions resulted in the formation of two liquid phases. The bottom layer was removed, and polyethylene glycol was added to it at a final concentration of 10 g/1. Again two liquid phases formed. The two top phases thus obtained were pooled and 1.6 volumes of cold acetone were added to the mixture. The precipitate formed was dissolved in water and desalted on a Sephadex G-25 column. The desalted protein solution was applied to a carboxymethyl-cellulose column equilibrated with 0.025 M sodium phosphate buffer of pH 6.0. Most of the proteins and the proteinases but none of the bacteriolytic enzymes passed the column unadsorbed. The column was washed with 0.05 M glycine-NaOH buffer of pH 8.8, whereupon the adsorbed bacteriolytic enzymes together with small amounts of proteinases and other proteins were eluted with 0.2 M ammonium carbonate. The material not adsorbed on the CM-cellulose column contained 22 % of the proteolytic activity of the initial cell-free solution and had a 26-fold higher specific activity. The enzyme solution eluted with carbonate contained 24 and 0.3 %, respectively, of the initial bacteriolytic and proteolytic activities. The specific activity of the bacteriolytic enzyme system was about 5000-fold higher than that of the original solution.     

Extracellular Lytic Enzymes of Myxococcus virescens

An easy and rapid method for the purification of a bacteriolytic endopeptidase produced by Myxococcus virescens is described. The bacteria were grown in casitone media and the cells were sedimented by centrifugation. About 1.2 g of montmorillonite were added per liter of cell-free culture solution. The clay was sedimented by centrifugation and the enzyme was then eluted by 0.05 M Na-phosphate buffer pH 6.0, containing 0.4 M NaCl. The enzyme was diluted with water and chromatographed on carboxymethyl-cellulose columns. The purified enzyme liberated free amino groups but no reducing sugars or N-acetylhexosamines when acting on purified N-acetylated cell walls of Micrococcus lysodeikticus. Analysis of N- and C-terminal amino acids in the digestion products showed that the enzyme had liberated about 110 nmoles of lysine ε-amino groups and 60 nmoles of alanine carboxyl groups per mg of cell wall. When it acted on a bisdisaccharide pentapeptide dimer isolated from M. lysodeikticus cell walls, it cleaved about 30% of the alanyl-lysine linkages. Consequently the enzyme was an alanyl-lysine endopeptidase. It had no muramyl-alanine amidase activity.     

Lytic effect of Bacillus subtilis elastase on gram-positive and negative bacteria.

Elastase of B. subtilis 6a caused lysis of freshly grown cells of Gram-negative (Proteus vulgaris, Klebsiella pneumoniae, Salmonella typhi and Pseudomonas aeruginosa and Gram-positive (B. subtilis) bacteria. Heat killed and lyophilised Gram-positive and negative bacteria showed higher sensitivity to elastase. Both Gram-negative and Gram-positive bacteria were lysed maximally by elastase at pH 8.0. At this pH, activity of elastase was maximum in Tris-HCl and glycine-NaOH buffers followed by Tris-maleate and cacodylate buffers.     

Extracellular Lytic Enzymes of Myxococcus virescens

Two bacteriolytic hexosaminidases isolated from Myxococcus virescens were characterized. When acting on purified cell walls of Micrococcus lysodeikticus they liberated reducing groups and N-acetylhexosamines. By chromatography on Sephadex G-50 and G-25 columns disaccharides were isolated from degraded cell walls. After reduction of the disaccharides with sodium borohydride the acid hydrolysis products were identified by thin layer chromatography. Only pale spots of glucosamine appeared after this treatment but the spots of muramic acid remained unchanged. The enzymes were found to be devoid of exo-β-N-acetylglucosaminidase activity. These results are compatible with the action of endo-β-N-acetylglucosaminidases.     

裂解性多糖单加氧酶及其应用研究进展北大核心

裂解多糖单加氧酶(lytic polysaccharide monooxygenases,LPMOs)是近几年新发现的氧化酶,该酶在生物质酶解方面发挥着重要的作用,因此,被描述为生物质解构助推器。LPMOs与底物的结合具有特异性,催化机理尚未完全阐明。虽然关于LPMOs的研究很多,但真正投入到工业生物质转化中的却很少,这对它们的表达、调控和应用都提出了挑战。本文首先系统综述了LPMOs的发现与分类、催化机制、构效关系,其次探讨了LPMOs的活性测定方法及重组表达技术,最后协同综述了LPMOs在不同领域的应用并对未来的研究方向进行了展望。本综述有助于加深对LPMOs的系统认识,推动LPMOs及其酶工程的研究,以期为LPMOs的研究和应用提供参考。     

Lytic activity of monomeric and oligomeric melittin

The haemolytic activities of melittin and melittin tetramer as induced by high phosphate counterion concentration, were monitored. Monomeric melittin was found to be fully lytic, whilst tetrameric melittin lacked such activity. Under conditions where melittin was fully tetrameric attempts were made to covalently cross-link the native tetramer using a series of different chain length bifunctional imido esters. The cross-linked oligomers were fully lytic under conditions where melittin was demonstrated to lack such activity. This finding, together with molecular weight determinations and circular dichroism studies, indicated that the cross-linked melittin was quite different to the native tetramer. The haemolytic activity of melittin-containing solutions was related to the concentration of monomeric melittin. The effect of reduced dielectric constant (?) on the aggregation behaviour of melittin and its derivatives was found to favour monomeric melittin.     

Membrane-Bound Lytic Endotransglycosylase in Escherichia coli

The gene for a novel endotype membrane-bound lytic transglycosylase, emtA, was mapped at 26.7 min of the E. coli chromosome. EmtA is a lipoprotein with an apparent molecular mass of 22 kDa. Overexpression of the emtA gene did not result in bacteriolysis in vivo, but the enzyme was shown to hydrolyze glycan strands isolated from murein by amidase treatment. The formation of tetra- and hexasaccharides, but no disaccharides, reflects the endospecificity of the enzyme. The products are characterized by the presence of 1,6-anhydromuramic acid, indicating a lytic transglycosylase reaction mechanism. EmtA may function as a formatting enzyme that trims the nascent murein strands produced by the murein synthesis machinery into proper sizes, or it may be involved in the formation of tightly controlled minor holes in the murein sacculus to facilitate the export of bulky compounds across the murein barrier.