Synthesis of the spore envelope in the developmental life cycle of Streptomyces coelicolor

Genus Streptomyces: Recent advances for biotechnological purposes

Actinomycetes are a distinct group of filamentous bacteria. The Streptomyces genus within this group has been extensively studied over the years, with substantial contributions to society and science. This genus is known for its antimicrobial production, as well as antitumor, biopesticide, and immunomodulatory properties. Therefore, the extraordinary plasticity of the Streptomyces genus has inspired new research techniques. The newest way of exploring Streptomyces has comprised the discovery of new natural metabolites and the application of emerging tools such as CRISPR technology in drug discovery. In this narrative review, we explore relevant published literature concerning the ongoing novelties of the Streptomyces genus.

On the evolution of natural product biosynthesis

Natural products are the raw material for drug discovery programmes. Bioactive natural products are used extensively in medicine and agriculture and have found utility as antibiotics, immunosuppressives, anti-cancer drugs and anthelminthics. Remarkably, the natural role and what mechanisms drive evolution of these molecules is relatively poorly understood. The exponential increase in genome and chemical data in recent years, coupled with technical advances in bioinformatics and genetics have enabled progress to be made in understanding the evolution of biosynthetic gene clusters and the products of their enzymatic machinery. Here we discuss the diversity of natural products, incorporating the mechanisms that govern evolution of metabolic pathways and how this can be applied to biosynthetic gene clusters. We build on the nomenclature of natural products in terms of primary, integrated, secondary and specialised metabolism and place this within an ecology-evolutionary-developmental biology framework. This eco-evo-devo framework we believe will help to clarify the nature and use of the term specialised metabolites in the future.

Persistence of Intracellular Bacterial Pathogens-With a Focus on the Metabolic Perspective

Persistence has evolved as a potent survival strategy to overcome adverse environmental conditions. This capability is common to almost all bacteria, including all human bacterial pathogens and likely connected to chronic infections caused by some of these pathogens. Although the majority of a bacterial cell population will be killed by the particular stressors, like antibiotics, oxygen and nitrogen radicals, nutrient starvation and others, a varying subpopulation (termed persisters) will withstand the stress situation and will be able to revive once the stress is removed. Several factors and pathways have been identified in the past that apparently favor the formation of persistence, such as various toxin/antitoxin modules or stringent response together with the alarmone (p)ppGpp. However, persistence can occur stochastically in few cells even of stress-free bacterial populations. Growth of these cells could then be induced by the stress conditions. In this review, we focus on the persister formation of human intracellular bacterial pathogens, some of which belong to the most successful persister producers but lack some or even all of the assumed persistence-triggering factors and pathways. We propose a mechanism for the persister formation of these bacterial pathogens which is based on their specific intracellular bipartite metabolism. We postulate that this mode of metabolism ultimately leads, under certain starvation conditions, to the stalling of DNA replication initiation which may be causative for the persister state.

Identification of the Wzx flippase, Wzy polymerase and sugar-modifying enzymes for spore coat polysaccharide biosynthesis in Myxococcus xanthus

The rod-shaped cells of Myxococcus xanthus, a Gram-negative deltaproteobacterium, differentiate to environmentally resistant spores upon starvation or chemical stress. The environmental resistance depends on a spore coat polysaccharide that is synthesised by the ExoA-I proteins, some of which are part of a Wzx/Wzy-dependent pathway for polysaccharide synthesis and export; however, key components of this pathway have remained unidentified. Here, we identify and characterise two additional loci encoding proteins with homology to enzymes involved in polysaccharide synthesis and export, as well as sugar modification and show that six of the proteins encoded by these loci are essential for the formation of environmentally resistant spores. Our data support that MXAN_3260, renamed ExoM and MXAN_3026, renamed ExoJ, are the Wzx flippase and Wzy polymerase, respectively, responsible for translocation and polymerisation of the repeat unit of the spore coat polysaccharide. Moreover, we provide evidence that three glycosyltransferases (MXAN_3027/ExoK, MXAN_3262/ExoO and MXAN_3263/ExoP) and a polysaccharide deacetylase (MXAN_3259/ExoL) are important for formation of the intact spore coat, while ExoE is the polyisoprenyl-phosphate hexose-1-phosphate transferase responsible for initiating repeat unit synthesis, likely by transferring N-acetylgalactosamine-1-P to undecaprenyl-phosphate. Together, our data generate a more complete model of the Exo pathway for spore coat polysaccharide biosynthesis and export.

Challenges and advances in genetic manipulation of filamentous actinomycetes - the remarkable producers of specialized metabolites

Covering: up to February 2019Actinomycetes are Gram positive bacteria of the phylum Actinobacteria. These organisms are one of the most important sources of structurally diverse, clinically used antibiotics and other valuable bioactive products, as well as biotechnologically relevant enzymes. Most strains were discovered by their ability to produce a given molecule and were often poorly characterized, physiologically and genetically. The development of genetic methods for Streptomyces and related filamentous actinomycetes has led to the successful manipulation of antibiotic biosynthesis to attain structural modification of microbial metabolites that would have been inaccessible by chemical means and improved production yields. Moreover, genome mining reveals that actinomycete genomes contain multiple biosynthetic gene clusters (BGCs), however only a few of them are expressed under standard laboratory conditions, leading to the production of the respective compound(s). Thus, to access and activate the so-called "silent" BGCs, to improve their biosynthetic potential and to discover novel natural products methodologies for genetic manipulation are required. Although different methods have been applied for many actinomycete strains, genetic engineering is still remaining very challenging for some "underexplored" and poorly characterized actinomycetes. This review summarizes the strategies developed to overcome the obstacles to genetic manipulation of actinomycetes and allowing thereby rational genetic engineering of this industrially relevant group of microorganisms. At the end of this review we give some tips to researchers with limited or no previous experience in genetic manipulation of actinomycetes. The article covers the most relevant literature published until February 2019.

Growth and differentiation properties of pikromycin-producing Streptomyces venezuelae ATCC15439

Streptomycetes naturally produce a variety of secondary metabolites, in the process of physiological differentiation. Streptomyces venezuelae differentiates into spores in liquid media, serving as a good model system for differentiation and a host for exogenous gene expression. Here, we report the growth and differentiation properties of S. venezuelae ATCC-15439 in liquid medium, which produces pikromycin, along with genome-wide gene expression profile. Comparison of growth properties on two media (SPA, MYM) revealed that the stationary phase cell viability rapidly decreased in SPA. Submerged spores showed partial resistance to lysozyme and heat, similar to what has been observed for better-characterized S. venezuelae ATCC10712, a chloramphenicol producer. TEM revealed that the differentiated cells in the submerged culture showed larger cell size, thinner cell wall than the aerial spores. We analyzed transcriptome profiles of cells grown in liquid MYM at various growth phases. During transition and/or stationary phases, many differentiationrelated genes were well expressed as judged by RNA level, except some genes forming hydrophobic coats in aerial mycelium. Since submerged spores showed thin cell wall and partial resistance to stresses, we examined cellular expression of MreB protein, an actin-like protein known to be required for spore wall synthesis in Streptomycetes. In contrast to aerial spores where MreB was localized in septa and spore cell wall, submerged spores showed no detectable signal. Therefore, even though the mreB transcripts are abundant in liquid medium, its protein level and/or its interaction with spore wall synthetic complex appear impaired, causing thinner- walled and less sturdy spores in liquid culture.

A Waking Review: Old and Novel Insights into the Spore Germination in Streptomyces

The complex development undergone by Streptomyces encompasses transitions from vegetative mycelial forms to reproductive aerial hyphae that differentiate into chains of single-celled spores. Whereas their mycelial life – connected with spore formation and antibiotic production – is deeply investigated, spore germination as the counterpoint in their life cycle has received much less attention. Still, germination represents a system of transformation from metabolic zero point to a new living lap. There are several aspects of germination that may attract our attention: (1) Dormant spores are strikingly well-prepared for the future metabolic restart; they possess stable transcriptome, hydrolytic enzymes, chaperones, and other required macromolecules stabilized in a trehalose milieu; (2) Germination itself is a specific sequence of events leading to a complete morphological remodeling that include spore swelling, cell wall reconstruction, and eventually germ tube emergences; (3) Still not fully unveiled are the strategies that enable the process, including a single cell’s signal transduction and gene expression control, as well as intercellular communication and the probability of germination across the whole population. This review summarizes our current knowledge about the germination process in Streptomyces, while focusing on the aforementioned points.

Crystal structure of Streptomyces coelicolor RraAS2, an unusual member of the RNase E inhibitor RraA protein family

Bacterial ribonuclease E (RNase E) plays a crucial role in the processing and decay of RNAs. A small protein named RraA negatively regulates the activity of RNase E via protein-protein interaction in various bacteria. Recently, RraAS1 and RraAS2, which are functional homologs of RraA from Escherichia coli, were identified in the Gram-positive species Streptomyces coelicolor. RraAS1 and RraAS2 inhibit RNase ES ribonuclease activity in S. coelicolor. RraAS1 and RraAS2 have a C-terminal extension region unlike typical bacterial RraA proteins. In this study, we present the crystal structure of RraAS2, exhibiting a hexamer arranged in a dimer of trimers, consistent with size exclusion chromatographic results. Importantly, the C-terminal extension region formed a long α-helix at the junction of the neighboring subunit, which is similar to the trimeric RraA orthologs from Saccharomyces cerevisiae. Truncation of the C-terminal extension region resulted in loss of RNase ES inhibition, demonstrating its crucial role. Our findings present the first bacterial RraA that has a hexameric assembly with a C-terminal extension α-helical region, which plays an essential role in the regulation of RNase ES activity in S. coelicolor.

Polydiglycosylphosphate Transferase PdtA (SCO2578) of Streptomyces coelicolor A3(2) Is Crucial for Proper Sporulation and Apical Tip Extension under Stress Conditions

Although anionic glycopolymers are crucial components of the Gram-positive cell envelope, the relevance of anionic glycopolymers for vegetative growth and morphological differentiation of Streptomyces coelicolor A3(2) is unknown. Here, we show that the LytR-CpsA-Psr (LCP) protein PdtA (SCO2578), a TagV-like glycopolymer transferase, has a dual function in the S. coelicolor A3(2) life cycle. Despite the presence of 10 additional LCP homologs, PdtA is crucial for proper sporulation. The integrity of the spore envelope was severely affected in a pdtA deletion mutant, resulting in 34% nonviable spores. pdtA deletion caused a significant reduction in the polydiglycosylphosphate content of the spore envelope. Beyond that, apical tip extension and normal branching of vegetative mycelium were severely impaired on high-salt medium. This growth defect coincided with the mislocalization of peptidoglycan synthesis. Thus, PdtA itself or the polydiglycosylphosphate attached to the peptidoglycan by the glycopolymer transferase PdtA also has a crucial function in apical tip extension of vegetative hyphae under stress conditions.

IMPORTANCE

Anionic glycopolymers are underappreciated components of the Gram-positive cell envelope. They provide rigidity to the cell wall and position extracellular enzymes involved in peptidoglycan remodeling. Although Streptomyces coelicolor A3(2), the model organism for bacterial antibiotic production, is known to produce two distinct cell wall-linked glycopolymers, teichulosonic acid and polydiglycosylphosphate, the role of these glycopolymers in the S. coelicolor A3(2) life cycle has not been addressed so far. This study reveals a crucial function of the anionic glycopolymer polydiglycosylphosphate for the growth and morphological differentiation of S. coelicolor A3(2). Polydiglycosylphosphate is attached to the spore wall by the LytR-CpsA-Psr protein PdtA (SCO2578), a component of the Streptomyces spore wall-synthesizing complex (SSSC), to ensure the integrity of the spore envelope. Surprisingly, PdtA also has a crucial role in vegetative growth under stress conditions and is required for proper peptidoglycan incorporation during apical tip extension.

A toolbox to measure changes in the cell wall glycopolymer composition during differentiation of Streptomyces coelicolor A3(2)

Cell wall glycopolymers (CWG) represent an important component of the Gram-positive cell envelope with many biological functions. The mycelial soil bacterium Streptomyces coelicolor A3(2) incorporates two distinct CWGs, polydiglycosylphosphate (PDP) and teichulosonic acid, into the cell wall of its vegetative mycelium but only little is known about their role in the complex life cycle of this microorganism. In this study we established assays to measure the total amount of CWGs in mycelial cell walls and spore walls, to quantify the individual CWGs and to determine the length of PDP. By applying these assays, we discovered that the relative amount of CWGs, especially of PDP, is reduced in spores compared to vegetative mycelium. Furthermore we found that PDP extracted from mycelial cell walls consisted of at least 19 repeating units, whereas spore walls contained substantially longer PDP polymers.

Control of Morphological Differentiation of Streptomyces coelicolor A3(2) by Phosphorylation of MreC and PBP2

During morphological differentiation of Streptomyces coelicolor A3(2), the sporogenic aerial hyphae are transformed into a chain of more than fifty spores in a highly coordinated manner. Synthesis of the thickened spore envelope is directed by the Streptomyces spore wall synthesizing complex SSSC which resembles the elongasome of rod-shaped bacteria. The SSSC includes the eukaryotic type serine/threonine protein kinase (eSTPK) PkaI, encoded within a cluster of five independently transcribed eSTPK genes (SCO4775-4779). To understand the role of PkaI in spore wall synthesis, we screened a S. coelicolor genomic library for PkaI interaction partners by bacterial two-hybrid analyses and identified several proteins with a documented role in sporulation. We inactivated pkaI and deleted the complete SCO4775-4779 cluster. Deletion of pkaI alone delayed sporulation and produced some aberrant spores. The five-fold mutant NLΔ4775-4779 had a more severe defect and produced 18% aberrant spores affected in the integrity of the spore envelope. Moreover, overbalancing phosphorylation activity by expressing a second copy of any of these kinases caused a similar defect. Following co-expression of pkaI with either mreC or pbp2 in E. coli, phosphorylation of MreC and PBP2 was demonstrated and multiple phosphosites were identified by LC-MS/MS. Our data suggest that elaborate protein phosphorylation controls activity of the SSSC to ensure proper sporulation by suppressing premature cross-wall synthesis.