Isolation and characterisation of a linked cluster of genes from Agrobacterium tumefaciens encoding proteins involved in flagellar basal-body structure

Exposure of Agrobacterium tumefaciens to agroinfiltration medium demonstrates cellular remodelling and may promote enhanced adaptability for molecular pharming

Agroinfiltration is used to treat plants with modified strains of Agrobacterium tumefaciens for the purpose of transient in planta expression of genes transferred from the bacterium. These genes encode valuable recombinant proteins for therapeutic or industrial applications. Treatment of large quantities of plants for industrial-scale protein production exposes bacteria (harboring genes of interest) to agroinfiltration medium that is devoid of nutrients and carbon sources for prolonged periods of time (possibly upwards of 24 h). Such conditions may negatively influence bacterial viability, infectivity of plant cells, and target protein production. Here, we explored the role of timing in bacterial culture preparation for agroinfiltration using mass spectrometry-based proteomics to define changes in cellular processes. We observed distinct profiles associated with bacterial treatment conditions and exposure timing, including significant changes in proteins involved in pathogenesis, motility, and nutrient acquisition systems as the bacteria adapt to the new environment. These data suggest a progression towards increased cellular remodelling over time. In addition, we described changes in growth- and environment-specific processes over time, underscoring the interconnectivity of pathogenesis and chemotaxis-associated proteins with transport and metabolism. Overall, our results have important implications for the production of transiently expressed target protein products, as prolonged exposure to agroinfiltration medium suggests remodelling of the bacterial proteins towards enhanced infection of plant cells.

Function and Regulation of Agrobacterium tumefaciens Cell Surface Structures that Promote Attachment

Agrobacterium tumefaciens attaches stably to plant host tissues and abiotic surfaces. During pathogenesis, physical attachment to the site of infection is a prerequisite to infection and horizontal gene transfer to the plant. Virulent and avirulent strains may also attach to plant tissue in more benign plant associations, and as with other soil microbes, to soil surfaces in the terrestrial environment. Although most A. tumefaciens virulence functions are encoded on the tumor-inducing plasmid, genes that direct general surface attachment are chromosomally encoded, and thus this process is not obligatorily tied to virulence, but is a more fundamental capacity. Several different cellular structures are known or suspected to contribute to the attachment process. The flagella influence surface attachment primarily via their propulsive activity, but control of their rotation during the transition to the attached state may be quite complex. A. tumefaciens produces several pili, including the Tad-type Ctp pili, and several plasmid-borne conjugal pili encoded by the Ti and At plasmids, as well as the so-called T-pilus, involved in interkingdom horizontal gene transfer. The Ctp pili promote reversible interactions with surfaces, whereas the conjugal and T-pili drive horizontal gene transfer (HGT) interactions with other cells and tissues. The T-pilus is likely to contribute to physical association with plant tissues during DNA transfer to plants. A. tumefaciens can synthesize a variety of polysaccharides including cellulose, curdlan (β-1,3 glucan), β-1,2 glucan (cyclic and linear), succinoglycan, and a localized polysaccharide(s) that is confined to a single cellular pole and is called the unipolar polysaccharide (UPP). Lipopolysaccharides are also in the outer leaflet of the outer membrane. Cellulose and curdlan production can influence attachment under certain conditions. The UPP is required for stable attachment under a range of conditions and on abiotic and biotic surfaces. Other factors that have been reported to play a role in attachment include the elusive protein called rhicadhesin. The process of surface attachment is under extensive regulatory control and can be modulated by environmental conditions, as well as by direct responses to surface contact. Complex transcriptional and post-transcriptional control circuitry underlies much of the production and deployment of these attachment functions.

A metabolic pathway leading to mannosylfructose biosynthesis in Agrobacterium tumefaciens uncovers a family of mannosyltransferases

A metabolic pathway for biosynthesis of the nonreducing disaccharide mannosylfructose (beta-fructofuranosyl-alpha-mannopyranoside), an important osmolyte in Agrobacterium tumefaciens, was discovered. We have identified and functionally characterized two ORFs that correspond to genes (named mfpsA and mfppA) encoding the rare enzymes mannosylfructose-phosphate synthase and mannosylfructose-phosphate phosphatase, an associated phosphohydrolase. The mfpsA and mfppA genes are arranged in an operon structure, whose transcription is up-regulated by NaCl, resulting in the accumulation of mannosylfructose in the cells. Not only is the biosynthesis of mannosylfructose mechanistically similar to that of sucrose, but the corresponding genes for the biosynthesis of both disaccharides are also phylogenetic close relatives. Importantly, a protein phylogeny analysis indicated that mannosylfructose-phosphate synthase defines a unique group of mannosyltransferases.

Investigations of Rhizobium biofilm formation

The development of nitrogen-fixing nodules of the Rhizobium-legume symbiosis, especially the early stages of root hair deformation and curling, infection thread formation, and nodule initiation, has been well studied from a genetic standpoint. In contrast, the factors important for the colonization of surfaces by rhizobia, including roots-an important prerequisite for nodule formation-have not been as thoroughly investigated. We developed conditions for analyzing the ability of two fast-growing rhizobia, Sinorhizobium meliloti and Rhizobium leguminosarum bv. viciae, to produce biofilms on abiotic surfaces such as glass, plastic microtiter plates, sand and soil as a prelude to characterizing the genes important for aggregation and attachment. Factors involved in adherence to abiotic surfaces are likely to be used in rhizobial attachment to legume root cells. In this report, we show that S. meliloti exopolysaccharide-deficient mutants as well as exopolysaccharide overproducers exhibit reduced biofilm phenotypes that show parallels with their nodulation abilities. We also investigated two flagella-less S. meliloti mutants and found them to have reduced biofilming capabilities. To investigate whether there was a symbiotic phenotype, we tested one of the Fla- mutants on two different S. meliloti hosts, alfalfa and white sweetclover, and found that nodule formation was significantly delayed on the latter.

Agrobacterium flagellar switch gene fliG is liquid inducible and important for virulence

Agrobacterium tumefaciens C58 was mutagenized with a mini-Tn5 transposon containing a promoterless gene encoding the green fluorescent protein (GFP). A mutant, CGS74, exhibited a higher GFP expression level in liquid media than on solid media. The ability of the mutant to cause tumors on plants was attenuated. Sequence analysis showed that the transposon was inserted at the fliG gene, which encodes a flagellar motor switch protein required for flagellar movement. Studies of the fliG-gfp fusion gene indicated that the promoter activity of the fliG gene was higher in liquid than in solid media. Electron microscopy studies demonstrated that the mutant was nonflagellate. This suggests that the A. tumefaciens motility is important for virulence and that bacterial flagellar synthesis occurs at a higher level in a liquid environment than in a solid environment, perhaps resulting in a higher motility.

A genomic sample sequence of the entomopathogenic bacterium Photorhabdus luminescens W14: potential implications for virulence

Photorhabdus luminescens is a pathogenic bacterium that lives in the guts of insect-pathogenic nematodes. After invasion of an insect host by a nematode, bacteria are released from the nematode gut and help kill the insect, in which both the bacteria and the nematodes subsequently replicate. However, the bacterial virulence factors associated with this "symbiosis of pathogens" remain largely obscure. In order to identify genes encoding potential virulence factors, we performed approximately 2,000 random sequencing reads from a P. luminescens W14 genomic library. We then compared the sequences obtained to sequences in existing gene databases and to the Escherichia coli K-12 genome sequence. Here we describe the different classes of potential virulence factors found. These factors include genes that putatively encode Tc insecticidal toxin complexes, Rtx-like toxins, proteases and lipases, colicin and pyocins, and various antibiotics. They also include a diverse array of secretion (e.g., type III), iron uptake, and lipopolysaccharide production systems. We speculate on the potential functions of each of these gene classes in insect infection and also examine the extent to which the invertebrate pathogen P. luminescens shares potential antivertebrate virulence factors. The implications for understanding both the biology of this insect pathogen and links between the evolution of vertebrate virulence factors and the evolution of invertebrate virulence factors are discussed.

A chemotaxis cluster from Agrobacterium tumefaciens

We report the DNA sequence of a 9.6-kb region of the Agrobacterium tumefaciens chromosome containing a putative 8-kb chemotaxis operon. The putative operon begins with orf1, whose predicted protein product shows strong sequence identity to methyl-accepting chemotaxis proteins (MCPs), followed by orf2, cheY1, cheA, cheR, cheB, cheY2, orf9, orf10. All of the identified homologues show a high degree of sequence conservation with their counterparts in the che operons from Sinorhizobium meliloti and Rhodobacter sphaeroides, and are arranged in a similar order. Mutations in orf1 and cheA result in impaired chemotaxis, whereas deletion of orf10, appears to have no effect on chemotaxis or motility. Although the putative operon does not contain a cheW homologue, heterologous probing and PCR using consensus primers indicates that cheW maps elsewhere in the Agrobacterium genome.