Isolation and identification of ribosyl-cis-zeatin from transfer RNA of Corynebacterium fascians

Bacterial terpenome

Covering: up to mid-2020 Terpenoids, also called isoprenoids, are the largest and most structurally diverse family of natural products. Found in all domains of life, there are over 80 000 known compounds. The majority of characterized terpenoids, which include some of the most well known, pharmaceutically relevant, and commercially valuable natural products, are produced by plants and fungi. Comparatively, terpenoids of bacterial origin are rare. This is counter-intuitive to the fact that recent microbial genomics revealed that almost all bacteria have the biosynthetic potential to create the C5 building blocks necessary for terpenoid biosynthesis. In this review, we catalogue terpenoids produced by bacteria. We collected 1062 natural products, consisting of both primary and secondary metabolites, and classified them into two major families and 55 distinct subfamilies. To highlight the structural and chemical space of bacterial terpenoids, we discuss their structures, biosynthesis, and biological activities. Although the bacterial terpenome is relatively small, it presents a fascinating dichotomy for future research. Similarities between bacterial and non-bacterial terpenoids and their biosynthetic pathways provides alternative model systems for detailed characterization while the abundance of novel skeletons, biosynthetic pathways, and bioactivies presents new opportunities for drug discovery, genome mining, and enzymology.

Analysis of genome sequences from plant pathogenic Rhodococcus reveals genetic novelties in virulence loci

Members of Gram-positive Actinobacteria cause economically important diseases to plants. Within the Rhodococcus genus, some members can cause growth deformities and persist as pathogens on a wide range of host plants. The current model predicts that phytopathogenic isolates require a cluster of three loci present on a linear plasmid, with the fas operon central to virulence. The Fas proteins synthesize, modify, and activate a mixture of growth regulating cytokinins, which cause a hormonal imbalance in plants, resulting in abnormal growth. We sequenced and compared the genomes of 20 isolates of Rhodococcus to gain insights into the mechanisms and evolution of virulence in these bacteria. Horizontal gene transfer was identified as critical but limited in the scale of virulence evolution, as few loci are conserved and exclusive to phytopathogenic isolates. Although the fas operon is present in most phytopathogenic isolates, it is absent from phytopathogenic isolate A21d2. Instead, this isolate has a horizontally acquired gene chimera that encodes a novel fusion protein with isopentyltransferase and phosphoribohydrolase domains, predicted to be capable of catalyzing and activating cytokinins, respectively. Cytokinin profiling of the archetypal D188 isolate revealed only one activate cytokinin type that was specifically synthesized in a fas-dependent manner. These results suggest that only the isopentenyladenine cytokinin type is synthesized and necessary for Rhodococcus phytopathogenicity, which is not consistent with the extant model stating that a mixture of cytokinins is necessary for Rhodococcus to cause leafy gall symptoms. In all, data indicate that only four horizontally acquired functions are sufficient to confer the trait of phytopathogenicity to members of the genetically diverse clade of Rhodococcus.

A successful bacterial coup d'état: how Rhodococcus fascians redirects plant development

Rhodococcus fascians is a gram-positive phytopathogen that induces differentiated galls, known as leafy galls, on a wide variety of plants, employing virulence genes located on a linear plasmid. The pathogenic strategy consists of the production of a mixture of six synergistically acting cytokinins that overwhelm the plant's homeostatic mechanisms, ensuring the activation of a signaling cascade that targets the plant cell cycle and directs the newly formed cells to differentiate into shoot meristems. The shoots that are formed upon infection remain immature and never convert to source tissues resulting in the establishment of a nutrient sink that is a niche for the epiphytic and endophytic R. fascians subpopulations. Niche formation is accompanied by modifications of the transcriptome, metabolome, physiology, and morphology of both host and pathogen. Here, we review a decade of research and set the outlines of the molecular basis of the leafy gall syndrome.

Rhodococcus fascians impacts plant development through the dynamic fas-mediated production of a cytokinin mix

The phytopathogenic actinomycete Rhodococcus fascians D188 relies mainly on the linear plasmid-encoded fas operon for its virulence. The bacteria secrete six cytokinin bases that synergistically redirect the developmental program of the plant to stimulate proliferation of young shoot tissue, thus establishing a leafy gall as a niche. A yeast-based cytokinin bioassay combined with cytokinin profiling of bacterial mutants revealed that the fas operon is essential for the enhanced production of isopentenyladenine, trans-zeatin, cis-zeatin, and the 2-methylthio derivatives of the zeatins. Cytokinin metabolite data and the demonstration of the enzymatic activities of FasD (isopentenyltransferase), FasE (cytokinin oxidase/dehydrogenase), and FasF (phosphoribohydrolase) led us to propose a pathway for the production of the cytokinin spectrum. Further evaluation of the pathogenicity of different fas mutants and of fas gene expression and cytokinin signal transduction upon infection implied that the secretion of the cytokinin mix is a highly dynamic process, with the consecutive production of a tom initiation wave followed by a maintenance flow.

Relationships between cytokinin production, presence of plasmids, and fasciation caused by strains of Corynebacterium fascians

Cytokinin activity in the culture medium of four pathogenic strains of Corynebacterium fascians varied from 168 to 0.4 mug of kinetin (6-furfurylaminopurine) equivalents per liter, as compared to 0.2 in an avirulent control. N(6)-Isopentenyladenine was the predominant cytokinin in the medium of all five strains, and its increased production was correlated with the degree of pathogenicity; however, the virulent strains also produced 8-13 times more cis-zeatin [6-(4-hydroxy-3-methyl-cis-2-butenylamino)purine] than the avirulent strain. The three most virulent strains (with the higher cytokinin contents in the medium) contained a large (M(r) approximately 10(8)) plasmid. The barely virulent strain contained a smaller plasmid. No plasmid was detected in the avirulent control. The total cytokinin content (mole%) and biological activity (mug of kinetin equivalent per mg of tRNA) in tRNA were about the same in all three virulent strains and in the avirulent control. Five cytokinins were isolated from each strain. Four were rigorously characterized as 6-(4-hydroxy-3-methyl-cis-2-butenylamino)-9-beta-D-ribofuranosylpurin e, 6-(3-methyl-2-butenylamino)-9-beta-D-ribofuranosylpurine, and their 2-methylthio derivatives. The fifth cytokinin was tentatively identified as 6-(4-hydroxy-3-methyl-trans-2-butenylamino)-9-beta-D-ribofuranosylpur ine. Ribosyl-cis-zeatin was 3-fold higher and N(6)-isopentenyladenosine was correspondingly lower in the plasmid-containing strains than in the plasmidless control. Because the entire syndrome of fasciation caused by infection can be induced with synthetic cytokinins, the disease would appear to be caused by plasmid-induced high rates of cytokinin production by the bacteria.

Presence of 2-methylthioribosyl-trans-zeatin in Azotobacter vinelandii tRNA

Hydroxylated cytokinin, 2-methylthio-N6-(4-hydroxy-3-methylbut-2-enyl) adenosine, was found in the tRNA of Azotobacter vinelandii. This cytokinin had the trans configuration, unlike the cis configuration reported for that from other bacteria. Culture-condition-dependent changes in the content of this thiocytokinin and a few other thionucleosides in the tRNA of this bacterium have been observed.

Distribution of cytokinin-active nucleosides in isoaccepting transfer ribonucleic acids from Agrobacterium tumefaciens

The cytokinin-active isoprenoid nucleosides of Agrobacterium tumefaciens transfer ribonucleic acid were identified by high-pressure liquid chromatography, permethylation, and mass spectroscopy. Besides the expected 6-[(3-methylbut-2-enyl)amino]-9-(beta-D-ribofuranosyl)purine (i6A) and its 2-methylthio derivative (ms2i6A), substantial amounts of cis- and trans-ribosylzeatin (io6A) and cis-2-(methylthio)ribosylzeatin (c-ms2io6A) were present. These hydroxylated side chain derivatives are normally characteristic of plant tRNA. Fractionation of the total bacterial tRNA on BD-cellulose and RPC-5 allowed isolation of purified iso-accepting species whose cytokinin nucleoside contents were then determined. Distribution of the isoprenoid nucleosides among the U-group tRNA species was not uniform. cis-Ribosylzeatin was found almost exclusively in one tRNASer while ms2io6A was found predominantly in tRNAPhe, tRNASer, and tRNATyr. Not all cytokinin-active species were found in every member of the U-group tRNAs. The only species present in tRNATrp was i6A; it contained no zeatin derivatives. The hydroxylation and methylthiolation processes appear to be highly specific and dependent upon tRNA structure or sequence.