Mechanisms and regulation of reduction-based iron uptake in plants

Despite the usually high abundance of iron (Fe) in soils, the low solubility of Fe-bearing minerals restricts the available Fe pools in most aerobic soils to levels that are far below those required for microbial or plant growth. To acquire the necessary amounts of Fe from the environment, organisms have evolved mechanisms that enhance the solubility and dissolution rate of Fe(iii) oxyhydroxides prevailing in aerobic soils. Chemically, these mechanisms are based on weakening of the Fe–O bond by reduction, chelation and protonation. Physiologically, two distinct and in all known cases mutually exclusive strategies can be distinguished: the excretion of siderophores capable of solubilizing external ferric Fe and subsequent uptake of the ferric siderophore complex; and reduction of Fe(iii) prior to uptake of the more soluble Fe²⁺ ion. With the exception of graminaceous species, in which Fe uptake is based on the former mechanism, the latter strategy is found in all cormophytes and certain algae, yeast and bacteria. In higher plants, the increase in their capacity to convert extracellular ferric to ferrous Fe is part of a series of physiological and morphological events that act in concert to achieve appropriate internal levels of Fe. It is this amalgam of features that determines the Fe efficiency of a species or cultivar that in turn affects the yield of economically important plants and the natural distribution of species. Adaptive changes to limited Fe availability have been studied at the molecular, physiological and whole-plant level. This review summarises current knowledge of the components of reduction-based Fe uptake in plants and presents an integrated view of the present understanding of mechanisms that control the rate and extent of Fe absorption by roots.     

The Neisseria meningitidis haemoglobin receptor: its role in iron utilization and virulence

The Neisseris meningitidis haemoglobin receptor gene, hmbR, was cloned by complementation in a porphyrin-requiring Escherichia coli mutant. hmbR encodes an 89.5 kDa outer membrane protein which shares amino acid homology with the TonB-dependent receptors of Gram-negative bacteria. HmbR had the highest similarity to Neisseria transferrin and lactoferrin receptors. The utilization of haemoglobin as an iron source required internalization of the haemin moiety by the cell. The mechanism of haemin internalization via the haemoglobin receptor was TonB-dependent in E. coli. A N. meningitidis hmbR mutant was unable to use haemoglobin but could still use haemin as a sole iron source. The existence of a second N. meningitidis receptor gene, specific for haemin, was shown by the isolation of cosmids which did not hybridize with the hmbR probe, but which were able to complement an E. coli hemA aroB mutant on haemin-supplemented plates. The N. meningitidis hmbR mutant was attenuated in an infant rat model for meningococcal infection, indicating that haemoglobin utilization is important for N. meningitidis virulence.     

Mechanisms of pathogenicity in mycobacteria

The purpose of this article is to review current knowledge about the mechanisms of pathogenicity of mycobacteria. The following aspects of the problem are discussed: chemically-defined compounds implicated in the mechanisms of pathogenicity; location in the cell wall of these compounds and their biological activities; mechanisms of intracellular survival of pathogenic mycobacteria as compared to intracellular killing of non-pathogenic mycobacteria; and pathogenesis of mycobacterial infection. The future prospects in the elucidation of the mechanisms of pathogenicity and their possible application for a better control of mycobacterial diseases are briefly discussed.     

Mechanisms and regulation of iron and heme utilization in Gram-negative bacteria

Iron and heme are essential nutrients for most pathogenic microorganisms and play a pivotal role in microbial pathogenesis. To survive within the iron-limited environment of the host, bacteria utilize iron-siderophore complexes, iron-binding proteins (transferrin, lactoferrin), free heme and heme bound to hemoproteins (hemoglobin, haptoglobin, hemopexin). A mechanism of iron and heme transport depends on the structures of Gram-negative bacterial membranes. Siderophores, hemophores and outer membrane receptors take part in iron or heme binding. The transport of these ligands across the outer membrane involves outer membrane receptors. The energy for this transport is delivered from the inner membrane by a TonB-ExbB-ExbD complex. The transport across the cytoplasmic membrane involves periplasmic and inner membrane proteins comprising the ABC systems, which utilize the energy derived from ATP hydrolysis. The major regulatory role in iron homeostasis plays a Fur-Fe2+ repressor.     

Autophagy: Mechanisms, regulation, and its role in tumorigenesis

Autophagy (from Greek “auto” — self, “phagos” — to eat) is the major catabolic process involved in the delivery and lysosomal degradation of long-lived intracellular components: proteins, lipids, nucleic acids, and organelles. Since the discovery of genes involved in regulation of autophagy in the 1990s, there has been a significant increase in studies of autophagy as a process involved in maintaining cellular homeostasis, as well as its role in the development of different pathologies. This review focuses on the basics of autophagy and its regulatory mechanisms. The role of autophagy in the maintenance of cellular homeostasis and tumorigenesis is also discussed.     

Tetracycline-inducible gene regulation in mycobacteria

A system for the tetracycline-inducible regulation of gene expression in mycobacteria has been developed. We have sub-cloned the tetRO region from the Corynebacterium glutamicum TetZ locus into a mycobacterial shuttle plasmid, making expression of genes cloned downstream of tetRO responsive to tetracycline. Using the luxAB-encoded luciferase from Vibrio harveyi as a reporter (pMind-Lx), we observed a 40-fold increase in light output from Mycobacterium smegmatis cultures 2 h after adding 20 ng ml⁻¹ of tetracycline. Similarly, exposure to the drug resulted in up to 20-fold increase in relative light units from M.bovis BCG carrying the reporter construct, and a 10-fold increase for M.tuberculosis. Tetracycline induction was demonstrated in log and stationary phase cultures. To evaluate whether this system is amenable to use in vivo, J774 macrophages were infected with M.bovis BCG[pMind-Lx], treated with amikacin to kill extracellular bacteria, and then incubated with tetracycline. A 10-fold increase in light output was measured after 24 h, indicating that intracellular bacteria are accessible and responsive to exogenously added tetracycline. To test the use of the tetracycline-inducible system for conditional gene silencing, mycobacteria were transformed with a pMind construct with tetRO driving expression of antisense RNA for the ftsZ gene. Bacterial cells containing the antisense construct formed filaments after 24 h exposure to tetracycline. These results demonstrate the potential of this tetracycline-regulated system for the manipulation of mycobacterial gene expression inside and outside cells.     

The dimycocerosate ester polyketide virulence factors of mycobacteria

Recent advances in the study of mycobacterial lipids indicate that the class of outer membrane lipids known as dimycocerosate esters (DIMs) are major virulence factors of clinically relevant mycobacteria including Mycobacterium tuberculosis and Mycobacterium leprae. DIMs are a structurally intriguing class of polyketide synthase-derived wax esters discovered over seventy years ago, yet, little was known until recently about their biosynthesis. Availability of several mycobacterial genomes has accelerated progress toward clarifying steps in the DIM biosynthetic pathway and it is our belief that reviewing the bases of our current knowledge will clarify outstanding issues and help direct future endeavors.     

Body-size regulation: combining genetics and physiology

New research has revealed that the activity of the insulin-signaling pathway in the prothoracic gland of Drosophila modulates ecdysone release and thereby influences both the duration and rate of larval growth.