Microorganisms are able to utilize nitrogen (N) from a wide range of organic and mineral compounds. In this paper, we review the current knowledge about the regulation of the enzyme systems involved in the acquisition of N and propose a conceptual model on the factors affecting the relative importance of organic and mineral N uptake. Most of the N input into soil is in the form of polymers, which first have to be broken down into smaller units by extracellular enzymes. The small organic molecules released by the enzymes can then be taken up directly or degraded further and the N taken up as ammonium (NH4+). When NH4+ is available at high concentrations, the utilization of alternative N sources, such as nitrate (NO3−) and organic molecules, is generally repressed. In contrast, when the NH4+ availability is low, enzyme systems for the acquisition of alternative N sources are de-repressed and the presence of a substrate can induce their synthesis. These mechanisms are known as N regulation. It is often assumed that most organic N is mineralized to NH4+ before uptake in soil. This pathway is generally known as the mineralization-immobilization-turnover (MIT) route. An advantage of the MIT route is that only one transporter system for N uptake is required. However, organic N uptake has the advantage that, in addition to N, it supplies energy and carbon (C) to sustain growth. Recent studies have shown that the direct uptake of organic molecules can significantly contribute to the N nutrition of soil microorganisms. We hypothesize that the relative importance of the direct and MIT route during the decomposition of residues is determined by three factors, namely the form of N available, the source of C, and the availability of N relative to C. The regulation system of soil microorganisms controls key steps in the soil N cycle and is central to determining the outcome of the competition for N between soil microorganisms and plants. More research is needed to determine the relative importance of the direct and MIT route in soil as well as the factors affecting the enzyme systems required for these two pathways.

Pathways of nitrogen utilization by soil microorganisms - a review.

Chitin is the second most abundant organic and renewable source in nature, after cellulose. Chitinases are chitin-degrading enzymes. Chitinases have important biophysiological functions and immense potential applications. In recent years, researches on fungal chitinases have made fast progress, especially in molecular levels. Therefore, the present review will focus on recent advances of fungal chitinases, containing their nomenclature and assays, purification and characterization, molecular cloning and expression, family and structure, regulation, and function and application.

Review of fungal chitinases.

Insect chitinases belong to family 18 glycosylhydrolases that hydrolyze chitin by an endo-type of cleavage while retaining the anomeric β-(1→4) configuration of products. There are multiple genes encoding chitinases and chitinase-like proteins in all insect species studied using bioinformatics searches. These chitinases differ in size, domain organization, physical, chemical and enzymatic properties, and in patterns of their expression during development. There are also differences in tissue specificity of expression. Based on a phylogenetic analysis, insect chitinases and chitinase-like proteins have been classified into several different groups. Results of RNA interference experiments demonstrate that at least some of these chitinases belonging to different groups serve non-redundant functions and are essential for insect survival, molting or development. Chitinases have been utilized for biological control of insect pests on transgenic plants either alone or in combination with other insecticidal proteins. Specific chitinases may prove to be useful as biocontrol agents and/or as vaccines.

Insect chitinase and chitinase-like proteins

Although many insects successfully live in dangerous environments exposed to diverse communities of microbes, they are often exploited and killed by specialist pathogens. Studies of host-pathogen interactions (HPI) provide valuable insights into the dynamics of the highly aggressive coevolutionary arms race between entomopathogenic fungi (EPF) and their arthropod hosts. The host defenses are designed to exclude the pathogen or mitigate the damage inflicted while the pathogen responds with immune evasion and utilization of host resources. EPF neutralize their immediate surroundings on the insect integument and benefit from the physiochemical properties of the cuticle and its compounds that exclude competing microbes. EPF also exhibit adaptations aimed at minimizing trauma that can be deleterious to both host and pathogen (eg, melanization of hemolymph), form narrow penetration pegs that alleviate host dehydration and produce blastospores that lack immunogenic sugars/enzymes but facilitate rapid assimilation of hemolymph nutrients. In response, insects deploy an extensive armory of hemocytes and macromolecules, such as lectins and phenoloxidase, that repel, immobilize, and kill EPF. New evidence suggests that immune bioactives work synergistically (eg, lysozyme with antimicrobial peptides) to combat infections. Some proteins, including transferrin and apolipophorin III, also demonstrate multifunctional properties, participating in metabolism, homeostasis, and pathogen recognition. This review discusses the molecular intricacies of these HPI, highlighting the interplay between immunity, stress management, and metabolism. Increased knowledge in this area could enhance the efficacy of EPF, ensuring their future in integrated pest management programs.

Entomopathogenic Fungi: New Insights into Host-Pathogen Interactions.

Stochastic mechano-chemical kinetics of molecular motors: A multidisciplinary enterprise from a physicist’s perspective

Chitinases were produced by Talaromyces flavus CGMCC 3.4301 when it was grown in the presence of chitin. Two chitinases from the culture filtrate of T. flavus were purified to homogeneity by fractional ammonium sulphate precipitation, ion-exchange chromatography on DEAE–Sepharose and Phenyl–Sepharose hydrophobic interaction chromatography. By SDS–PAGE, the molecular weight of the two enzymes was estimated to be 41 and 32 kDa, respectively. The 41 kDa chitinase (CHIT41) had a 4.0 pH optimum; the 32 kDa chitinase (CHIT32) optimum activity was at pH 5.0. The optimum temperature for the two chitinase activities was 40 °C. The two chitinases had activity against cell wall of Verticilliumdahliae, Sclerotiniasclerotiorum and Rhizoctonia solani, and inhibited spore germination and germ tube elongation of Alternaria alternata, Fusarium moniliforme, and Magnaporthe grisea.

Purification and partial characterization of two chitinases from the mycoparasitic fungus Talaromyces flavus.

Tpb55 is an antagonistic bacteria strain isolated from tobacco phyllosphere.The strain was identified as Bacillus subtilis on the basis of morphological,physiological and biochemical characters as well as sequence analysis of 16S-23S rDNA ITS.The strain is bacilliform,Gram positive,and spore forming,consistent to B.subtilis according to its physiological and biochemical characters.The homology of 16S-23S rDNA ITS sequences between Tpb55 and reference strain was 99.75%.Further studies indicated that its metabolic products showed broad antagonistic spectra.The antagonistic activity was heat-stable,insensitive to pH,resistant to proteolytic enzymes,and freely soluble in water.Tpb55 may be of potential for biological control of plant diseases.

Li DuoChuan

Papers

Review of fungal chitinases.

Purification and partial characterization of two chitinases from the mycoparasitic fungus Talaromyces flavus.

Identification and Antagonistic Activity of Tobacco Phyllosphere Bacteria Strain Tpb55

Cloning and bioinformatics analysis of chitinase gene from mycoparasitic Talaromyces flavus.

Cloning of glucoamylase gene (gla) from Thermomyces lanuginosus and its expression in Pichia pastoris.