The Streptomyces genus is well known for its ability to produce bio-active secondary metabolites of great medical interest. However, the metabolic features accompanying these bio-productions remain to be defined. In this study, the comparison of related model strains producing differing levels of actinorhoddin (ACT), showed that S. lividans, a weak producer, had high TriAcylGlycerol (TAG) content indicative of a glycolytic metabolism. In contrast, the strong producer, S. coelicolor, was characterized by low TAG content, active consumption of its polyphosphate (PolyP) stores and extremely high ATP/ADP ratios. This indicated highly active oxidative metabolism that was correlated with induction of ACT biosynthesis. Interestingly, in conditions of phosphate limitation, the ppk mutant had TAG content and ACT production levels intermediary between those of S. lividans and S. coelicolor. This strain was characterized by high ADP levels indicating that Ppk was acting as an Adenosine Di Phosphate Kinase. Its absence resulted in energetic stress that is proposed to trigger an activation of oxidative metabolism to restore its energetic balance. This process, which is correlated with ACT biosynthesis, requires acetylCoA to fuel the Krebs cycle and phosphate for ATP generation by the ATP synthase coupled to the respiratory chain, resulting in low TAG and polyP content of the ACT producing strains.

φRSM1 and φRSM3 (φRSM phages) are filamentous phages (inoviruses) that infect Ralstonia solanacearum, the causative agent of bacterial wilt. Infection by φRSM phages causes several cultural and physiological changes to host cells, especially loss of virulence. In this study, we characterized changes related to the virulence in φRSM3-infected cells, including (i) reduced twitching motility and reduced amounts of type IV pili (Tfp), (ii) lower levels of β-1,4-endoglucanase (Egl) activity and extracellular polysaccharides (EPS) production, and (iii) reduced expression of certain genes (egl, pehC, phcA, phcB, pilT, and hrpB). The significantly lower levels of phcA and phcB expression in φRSM3-infected cells suggested that functional PhcA was insufficient to activate many virulence genes. Tomato plants injected with φRSM3-infected cells of different R. solanacearum strains did not show wilting symptoms. The virulence and virulence factors were restored when φRSM3-encoded orf15, the gene for a putative repressor-like protein, was disrupted. Expression levels of phcA as well as other virulence-related genes in φRSM3-ΔORF15-infected cells were comparable with those in wild-type cells, suggesting that orf15 of φRSM3 may repress phcA and, consequently, result in loss of virulence.

Two prophages, called varphiRSM3 and varphiRSM4, that are closely related to, but differ from, filamentous phage varphiRSM1, have been detected in strains of the Ralstonia solanacearum species complex. The prophage varphiRSM3, found in host strain MAFF730139, could be converted to infectious phage by means of PCR and transfection. The nucleotide sequence of varphiRSM3 is highly conserved relative to varphiRSM1 except for open reading frame 2 (ORF2), encoding an unknown protein, and ORF9 encoding the presumed adsorption protein that determines host range. The two host ranges differ dramatically and correlate closely with different gel electrophoresis banding patterns for cell surface fimbriae. Infections by varphiRSM1 and varphiRSM3 enhance bacterial cell aggregation and reduce the bacterial host virulence in tomato plants. Database searches in the R. solanacearum strains of known genomic sequence revealed two inovirus prophages, one designated varphiRSM4 that is homologous to varphiRSM1 and varphiRSM3, and one homologues to RSS1, in the genome of strain UW551.

Ralstonia solanacearum is the causative agent of bacterial wilt in many important crops. ϕRSS1 is a filamentous phage that infects R. solanacearum strains. Upon infection, it alters the physiological state and the behavior of host cells. Here, we show that R. solanacearum infected by ϕRSS1 becomes more virulent on host plants. Some virulence and pathogenicity factors, such as extracellular polysaccharide (EPS) synthesis and twitching motility, increased in the bacterial host cells infected with ϕRSS1, resulting in early wilting. Tomato plants inoculated with ϕRSS1-infected bacteria wilted 2 to 3 days earlier than those inoculated with wild-type bacteria. Infection with ϕRSS1 induced early expression of phcA, the global virulence regulator. phcA expression was detected in ϕRSS1-infected cells at cell density as low as 10(4) CFU/ml. Filamentous phages are assembled on the host cell surface and many phage particles accumulate on the cell surface. These surface-associated phage particles (phage proteins) may change the cell surface nature (hydrophobicity) to give high local cell densities. ϕRSS1 infection also enhanced PilA and type IV pilin production, resulting in increased twitching motility.

Jumbo phages infecting Ralstonia solanacearum were isolated in Thailand (ϕRSL2) and Japan (ϕRSF1). They were similar regarding virion morphology, genomic arrangement, and host range. Phylogenetic and proteomic tree analyses demonstrate that the ϕRSL2 and ϕRSF1 belong to a group of evolutionary related phages, including Pseudomonas phages ϕKZ, 201ϕ2-1 and all previously described ϕKZ-related phages. Despite conserved genomic co-linearity between the ϕRSL2 and ϕRSF1, they differ in protein separation patterns. A major difference was seen in the detection of virion-associated-RNA polymerase subunits. All β- and β'-subunits were detected in ϕRSF1, but one β'-subunit was undetected in ϕRSL2. Furthermore, ϕRSF1 infected host cells faster (latent period: 60 and 150min for ϕRSF1 and ϕRSL2, respectively) and more efficiently than ϕRSL2. Therefore, the difference in virion-associated-RNA polymerase may affect infection efficiency. Finally, we show that ϕRSF1 is able to inhibit bacterial wilt progression in tomato plants.

Xanthomonas virus (phage) XacN1 is a novel jumbo myovirus infecting Xanthomonas citri, the causative agent of Asian citrus canker. Its linear 384,670 bp double-stranded DNA genome encodes 592 proteins and presents the longest (66 kbp) direct terminal repeats (DTRs) among sequenced viral genomes. The DTRs harbor 56 tRNA genes, which correspond to all 20 amino acids and represent the largest number of tRNA genes reported in a viral genome. Codon usage analysis revealed a propensity for the phage encoded tRNAs to target codons that are highly used by the phage but less frequently by its host. The existence of these tRNA genes and seven additional translation-related genes as well as a chaperonin gene found in the XacN1 genome suggests a relative independence of phage replication on host molecular machinery, leading to a prediction of a wide host range for this jumbo phage. We confirmed the prediction by showing a wider host range of XacN1 than other X. citri phages in an infection test against a panel of host strains. Phylogenetic analyses revealed a clade of phages composed of XacN1 and ten other jumbo phages, indicating an evolutionary stable large genome size for this group of phages.

In this study, filamentous phage XacF1, which can infect Xanthomonas axonopodis pv. citri (Xac) strains, was isolated and characterized. Electron microscopy showed that XacF1 is a member of the family Inoviridae and is about 600 nm long. The genome of XacF1 is 7325 nucleotides in size, containing 13 predicted open reading frames (ORFs), some of which showed significant homology to Ff-like phage proteins such as ORF1 (pII), ORF2 (pV), ORF6 (pIII), and ORF8 (pVI). XacF1 showed a relatively wide host range, infecting seven out of 11 strains tested in this study. Frequently, XacF1 was found to be integrated into the genome of Xac strains. This integration occurred at the host dif site (attB) and was mediated by the host XerC/D recombination system. The attP sequence was identical to that of Xanthomonas phage Cf1c. Interestingly, infection by XacF1 phage caused several physiological changes to the bacterial host cells, including lower levels of extracellular polysaccharide production, reduced motility, slower growth rate, and a dramatic reduction in virulence. In particular, the reduction in virulence suggested possible utilization of XacF1 as a biological control agent against citrus canker disease.

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