No abstract

No abstract

Gliotoxin, and other related molecules, are encoded by multi-gene clusters and biosynthesized by fungi using non-ribosomal biosynthetic mechanisms. Almost universally described in terms of its toxicity towards mammalian cells, gliotoxin has come to be considered as a component of the virulence arsenal of Aspergillus fumigatus. Here we show that deletion of a single gene, gliT, in the gliotoxin biosynthetic cluster of two A. fumigatus strains, rendered the organism highly sensitive to exogenous gliotoxin and completely disrupted gliotoxin secretion. Addition of glutathione to both A. fumigatus ΔgliT strains relieved gliotoxin inhibition. Moreover, expression of gliT appears to be independently regulated compared to all other cluster components and is up-regulated by exogenous gliotoxin presence, at both the transcript and protein level. Upon gliotoxin exposure, gliT is also expressed in A. fumigatus ΔgliZ, which cannot express any other genes in the gliotoxin biosynthetic cluster, indicating that gliT is primarily responsible for protecting this strain against exogenous gliotoxin. GliT exhibits a gliotoxin reductase activity up to 9 µM gliotoxin and appears to prevent irreversible depletion of intracellular glutathione stores by reduction of the oxidized form of gliotoxin. Cross-species resistance to exogenous gliotoxin is acquired by A. nidulans and Saccharomyces cerevisiae, respectively, when transformed with gliT. We hypothesise that the primary role of gliotoxin may be as an antioxidant and that in addition to GliT functionality, gliotoxin secretion may be a component of an auto-protective mechanism, deployed by A. fumigatus to protect itself against this potent biomolecule.

Maximal expression of the histidine operon of Salmonella typhimurium in a coupled in vitro transcription-translation system is strongly dependent upon addition of guanosine 5'-diphosphate 3'-diphosphate (ppGpp). This requirement for ppGpp is exerted at the level of transcription through a mechanism distinct from the his-operon-specific regulatory mechanism. In vivo derepression of the his operon is markedly defective when histidine starvation is imposed on a relA mutant-unable to rapidly increase synthesis of ppGpp-growing in amino-acid-rich medium. Increased sensitivity of relA mutants to growth inhibition by a number of amino-acid analogs suggests that pp-pp is generally important in adjusting expression of amino-acid-producing systems. Analysis of these findings leads us to propose that ppcpp is a positive effector in a system that enables the cell to balance endogenous amino-acid production with environmental conditions of amino-acid availability, and to compensate efficiently for transient changes in these conditions. We propose a unifying theory of the role of ppGpp as the general signal molecule (alarmone) in a "super-contro " which senses an amino-acid deficiency and redirects the cell's economy in response.The "stringent phenomenon" in bacteria (1) has been primarily interpreted as being a regulatory mechanism for adjusting the rate of synthesis of stable RNA (i.e., rRNA and tRNA) with respect to the availability of amino acids for protein synthesis. Evidence has accumulated suggesting that the unusual nucleotide guanosine 5'-diphosphate 3'-diphosphate (ppGpp)-synthesized by the relA gene product on the ribosome as a function of tRNA charging (2)-acts as a negative effector in this regulation, although the precise mechanism is unclear. Comparison of rel+ and relA bacterial strains has revealed that ppGpp also may negatively control other aspects of cellular metabolism: biosynthesis of lipids (3), nucleotides (4), and polyamines (5); and uptake of purines and pyrimidines (6, 7).In this paper we show that ppGpp is also a positive effector for transcription of the histidine operon of Salmonella typhimurium, and that it apparently positively regulates production of other amino acids.MATERIALS AND METHODS Bacterial Strains. Isogenic pairs of S. typhimurium strains were employed for the purpose indicated: TA471 (his AOGDCBH2253 hisT1504) and TA705 (his-AOGDCBH2253 hisT1504 relAl)-preparation of in vitro protein synthesizing extracts; TA2383 (hisG46 dhuAl) and TA2384 (hisG46 dhuAl relAl)-in vivo his operon derepression experiments; and TA1995 (dhuAl) and TA1996 (dhuAl relAl)-amino-acid analog studies. The rel-1 mutation (8) has been designated a relA mutation (9) on the basis of genetic mapping, phenotype, and biochemical characterization (unpublished experiments). The dhuAl mutation (10) was used to facilitate strain construction and is not pertinent to these studies. The following Escherichia coli lysogens (11) were used as source of template DNA: TA1933

There is a continuing rise in the occurrence of multidrug-resistant bacterial infections. Antibiotic resistance to currently available antibiotics has become a global health issue leading to an urgent need for alternative antibacterial strategies. There has been a renewed interest in the development of antibacterial agents from natural sources, and trans-cinnamaldehyde is an example of a naturally occurring compound that has received significant attention in recent years. Trans-Cinnamaldehyde has been shown to possess substantial antimicrobial activity, as well as an array of other medicinal properties, and represents an intriguing hit compound from which a number of derivatives have been developed. In some cases, these derivatives have been shown to possess improved activity, not only compared to trans-cinnamaldehyde but also to commonly used antibiotics. Therefore, understanding the antibacterial mechanisms of action that these compounds elicit is imperative in order to facilitate their development and the development of new antibacterial agents that could exploit similar mechanistic approaches. The purpose of this review is to provide an overview of current knowledge on the antibacterial activity and mechanisms of action of cinnamaldehyde and its derivatives, and to highlight significant contributions made in this research area. It is hoped that the findings presented in this work will aid the future development of new antibacterial agents.