Evidence and characterization of a gene cluster required for the production of viscosin, a lipopeptide biosurfactant, by a strain ofPseudomonas fluorescens

Review lipopeptides biosurfactants: Mean classes and new insights for industrial, biomedical, and environmental applications

Lipopeptides are microbial surface active compounds produced by a wide variety of bacteria, fungi, and yeast. They are characterized by high structural diversity and have the ability to decrease the surface and interfacial tension at the surface and interface, respectively. Surfactin, iturin, and fengycin of Bacillus subtilis are among the most popular lipopeptides. Lipopepetides can be applied in diverse domains as food and cosmetic industries for their emulsification/de-emulsification capacity, dispersing, foaming, moisturizing, and dispersing properties. Also, they are qualified as viscosity reducers, hydrocarbon solubilizing and mobilizing agents, and metal sequestering candidates for application in environment and bioremediation. Moreover, their ability to form pores and destabilize biological membrane permits their use as antimicrobial, hemolytic, antiviral, antitumor, and insecticide agents. Furthermore, lipopeptides can act at the surface and can modulate enzymes activity permitting the enhancement of the activity of certain enzymes ameliorating microbial process or the inhibition of certain other enzymes permitting their use as antifungal agents. This article will present a detailed classification of lipopeptides biosurfactant along with their producing strain and biological activities and will discuss their functional properties and related applications.

Genetic regulations of the biosynthesis of microbial surfactants: an overview

Microbial biosurfactants are surface active metabolites synthesized by microbes growing on a variety of substrates. In spite of having great potential for commercial, therapeutic and environmental applications, industrial level production has not been realized for their low yields and productivities. One vital factor determining their biosynthesis is the genetic makeup of the producer organisms. Studies on molecular genetics and biochemistry of the synthesis of several biosurfactants have revealed the operons, the enzymes and the metabolic pathways required for their extracellular production. Surfactin, a cyclic lipopeptide biosurfactant is a potent antimicrobial agent and is produced as a result of non-ribosomal biosynthesis catalyzed by a large multienzyme peptide synthetase complex called the surfactin synthetase. Pathways for the synthesis of other lipopeptides such as iturin, lichenysin and arthrofactin are also mediated by similar enzyme complexes. These non-ribosomal peptide synthetases (NRPSs) responsible for lipopeptide biosynthesis display a high degree of structural similarity among themselves even from distant microbial species. Plasmid-encoded- rhlA, B, R and I genes of rhl quorum sensing system are required for production of glycolipid biosurfactants by Pseudomonas species. Molecular genetics of biosynthesis of alasan and emulsan by Acinetobacter species and of the fungal biosurfactants such as mannosylerythritol lipids (MEL) and hydrophobins have been deciphered. However, limited genetic information is available about biosynthesis of other biosurfactants such as viscosin, amphisin and putisolvin produced by some strains of Pseudomonas species. Understanding of the genetic regulatory mechanisms would help to develop metabolically engineered hyper-producing strains with better product characteristics and acquired capability of utilizing cheap agro-industrial wastes as substrates. This article thus provides an overview of the role and importance of molecular genetics and gene regulation mechanisms behind the biosynthesis of various microbial surfactants of commercial importance.

Diversity and functional analysis of LuxR-type transcriptional regulators of cyclic lipopeptide biosynthesis in Pseudomonas fluorescens

Cyclic lipopeptides (CLPs) are produced by many Pseudomonas species and have several biological functions, including a role in surface motility, biofilm formation, virulence, and antimicrobial activity. This study focused on the diversity and role of LuxR-type transcriptional regulators in CLP biosynthesis in Pseudomonas species and, specifically, viscosin production by Pseudomonas fluorescens strain SBW25. Phylogenetic analyses showed that CLP biosynthesis genes in Pseudomonas strains are flanked by LuxR-type regulators that contain a DNA-binding helix-turn-helix domain but lack N-acylhomoserine lactone-binding or response regulator domains. For SBW25, site-directed mutagenesis of the genes coding for either of the two identified LuxR-type regulators, designated ViscAR and ViscBCR, strongly reduced transcript levels of the viscABC biosynthesis genes and resulted in a loss of viscosin production. Expression analyses further showed that a mutation in either viscAR or viscBCR did not substantially (change of <2.5-fold) affect transcription of the other regulator. Transformation of the DeltaviscAR mutant of SBW25 with a LuxR-type regulatory gene from P. fluorescens strain SS101 that produces massetolide, a CLP structurally related to viscosin, restored transcription of the viscABC genes and viscosin production. The results further showed that a functional viscAR gene was required for heterologous expression of the massetolide biosynthesis genes of strain SS101 in strain SBW25, leading to the production of both viscosin and massetolide. Collectively, these results indicate that the regulators flanking the CLP biosynthesis genes in Pseudomonas species represent a unique LuxR subfamily of proteins and that viscosin biosynthesis in P. fluorescens SBW25 is controlled by two LuxR-type transcriptional regulators.

Genes involved in cyclic lipopeptide production are important for seed and straw colonization by Pseudomonas sp. strain DSS73

Survival in natural bulk soil and colonization of sugar beet seeds and barley straw residues were determined for Pseudomonas sp. strain DSS73 and Tn5 mutants in amsY (encoding a peptide synthetase involved in production of the cyclic lipopeptide amphisin) and gacS (encoding the sensory kinase of the two-component GacA/GacS regulatory system). No differences in survival or growth in response to carbon amendment (citrate) were observed in bulk soil. However, both mutants were impaired in their colonization of sugar beet seeds and barley straw residues by an inoculum established in the bulk soil. The two mutants had comparable colonization phenotypes, suggesting that amphisin production is more important for colonization than other gacS-controlled traits.

Plant perceptions of plant growth-promoting Pseudomonas

Plant-associated Pseudomonas live as saprophytes and parasites on plant surfaces and inside plant tissues. Many plant-associated Pseudomonas promote plant growth by suppressing pathogenic micro-organisms, synthesizing growth-stimulating plant hormones and promoting increased plant disease resistance. Others inhibit plant growth and cause disease symptoms ranging from rot and necrosis through to developmental dystrophies such as galls. It is not easy to draw a clear distinction between pathogenic and plant growth-promoting Pseudomonas. They colonize the same ecological niches and possess similar mechanisms for plant colonization. Pathogenic, saprophytic and plant growth-promoting strains are often found within the same species, and the incidence and severity of Pseudomonas diseases are affected by environmental factors and host-specific interactions. Plants are faced with the challenge of how to recognize and exclude pathogens that pose a genuine threat, while tolerating more benign organisms. This review examines Pseudomonas from a plant perspective, focusing in particular on the question of how plants perceive and are affected by saprophytic and plant growth-promoting Pseudomonas (PGPP), in contrast to their interactions with plant pathogenic Pseudomonas. A better understanding of the molecular basis of plant-PGPP interactions and of the key differences between pathogens and PGPP will enable researchers to make more informed decisions in designing integrated disease-control strategies and in selecting, modifying and using PGPP for plant growth promotion, bioremediation and biocontrol.