African swine fever virus (ASFV) is a complex DNA virus that employs polyprotein processing at Gly-GlyXaa sites as a strategy to produce several major core components of the viral particle. The virus gene S273R encodes a 31-kDa protein that contains a "core domain" with the conserved catalytic residues characteristic of SUMO-1-specific proteases and the adenovirus protease. Using a COS cell expression system, it was found that protein pS273R is capable of cleaving the viral polyproteins pp62 and pp220 in a specific way giving rise to the same intermediates and mature products as those produced in ASFV-infected cells. Furthermore, protein pS273R, like adenovirus protease and SUMO-1-specific enzymes, is a cysteine protease, because its activity is abolished by mutation of the predicted catalytic histidine and cysteine residues and is inhibited by sulfhydryl-blocking reagents. Protein pS273R is expressed late after infection and is localized in the cytoplasmic viral factories, where it is found associated with virus precursors and mature virions. In the virions, the protein is present in the core shell, a domain where the products of the viral polyproteins are also located. The identification of the ASFV protease will allow a better understanding of the role of polyprotein processing in virus assembly and may contribute to our knowledge of the emerging family of SUMO-1-specific proteases.Positive strand RNA viruses and retroviruses encode polyproteins, which are proteolytically cleaved by viral proteases to yield the nonstructural and structural proteins required for replication and morphogenesis (1-3). On the other hand, DNA viruses, such as adenoviruses and poxviruses, synthesize precursor proteins whose maturation by proteolytic removal of terminal peptides plays an essential role in virion formation (1).African swine fever virus (ASFV), 1 a large and complex virus containing a 170-kb double-stranded DNA molecule with 151 potential genes (4, 5), is atypical among DNA viruses in that it encodes two polyproteins, pp220 and pp62, which are cleaved to produce six major structural components of the virus particle (6, 7). These proteins, p150, p37, p34, and p14, derived from polyprotein pp220 and p35 and p15, products of polyprotein pp62, are the major components of the core shell, a thick protein layer that surrounds the DNA-containing central nucleoid and that is enwrapped by the inner lipoprotein envelope and the icosahedral capsid (8).2 All the proteolytic cleavages occur after the second Gly of the consensus sequence Gly-GlyXaa, which is also recognized as a cleavage site in the maturation of adenovirus structural proteins and in some cellular proteins, including polyubiquitin (9) and ubiquitin-like proteins (10). A similar cleavage site (Ala-Gly-Xaa) is used for the maturation of vaccinia virus structural proteins (11,12). Although the adenovirus protease that processes at Gly-Gly-Xaa sites is well characterized (13-15), the enzymes involved in the processing of the ASFV polyproteins or in the cleavage of vaccin...

The replication of many RNA viruses involves the translation of polyproteins, whose processing by endopeptidases is a critical step for the release of functional subunits. P1 is the first protease encoded in plant potyvirus genomes; once activated by an as-yet-unknown host factor, it acts in cis on its own C-terminal end, hydrolyzing the P1-HCPro junction. Earlier research suggests that P1 cooperates with HCPro to inhibit host RNA silencing defenses. Using Plum pox virus as a model, we show that although P1 does not have a major direct role in RNA silencing suppression, it can indeed modulate HCPro function by its self-cleavage activity. To study P1 protease regulation, we used bioinformatic analysis and in vitro activity experiments to map the core C-terminal catalytic domain. We present evidence that the hypervariable region that precedes the protease domain is predicted as intrinsically disordered, and that it behaves as a negative regulator of P1 proteolytic activity in in vitro cleavage assays. In viral infections, removal of the P1 protease antagonistic regulator is associated with greater symptom severity, induction of salicylate-dependent pathogenesis-related proteins, and reduced viral loads. We suggest that fine modulation of a viral protease activity has evolved to keep viral amplification below host-detrimental levels, and thus to maintain higher long-term replicative capacity.

This report shows that African swine fever virus (ASFV)‐‐a large DNA‐containing virus‐‐synthesizes a polyprotein to produce several of its structural proteins. By immunoprecipitation analysis, we have found that ASFV polyprotein is a 220 kDa myristoylated polypeptide (pp220) which, after proteolytic processing, gives rise to four major structural proteins: p150, p37, p34 and p14. Processing of the ASFV polyprotein takes place at the consensus sequence Gly‐Gly‐X and occurs through an ordered cascade of proteolytic cleavages. So far, polyprotein processing as a mechanism of gene expression had been found only in positive‐strand RNA viruses and retroviruses. According to the results presented here, ASFV is the first example of a DNA virus that synthesizes a polyprotein as a strategy of gene expression.

Since the discovery of microRNA (miRNA)-guided processing, a new type of RNA silencing, the possibility that such a mechanism could play a role in virus defense has been proposed. In this work, we have analyzed whether Plum pox virus (PPV) chimeras bearing miRNA target sequences (miR171, miR167, and miR159), which have been reported to be functional in Arabidopsis, were affected by miRNA function in three different host plants. Some of these PPV chimeras had clearly impaired infectivity compared with those carrying nonfunctional miRNA target sequences. The behaviors of PPV chimeras were similar but not identical in all the plants tested, and the deleterious effect on virus infectivity depended on the miRNA sequence cloned and on the site of insertion in the viral genome. The effect of the miRNA target sequence was drastically alleviated in transgenic plants expressing the silencing suppressor P1/HCPro. Furthermore, we show that virus chimeras readily escape RNA silencing interference through mutations within the miRNA target sequence, which mainly affected nucleotides matching the 5-terminal region of the miRNA.

Polyprotein processing is a common strategy of gene expression in many positive-strand RNA viruses and retroviruses but not in DNA viruses. African swine fever virus (ASFV) is an exception because it encodes a polyprotein, named pp220, to produce several major components of the virus particle, proteins p150, p37, p34, and p14. In this study, we analyzed the assembly pathway of ASFV and the contribution of the polyprotein products to the virus structure. Electron microscopic studies revealed that virions assemble from membranous structures present in the viral factories. Viral membranes became polyhedral immature virions after capsid formation on their convex surface. Beneath the lipid envelope, two distinct domains appeared to assemble consecutively: first a thick protein layer that we refer to as core shell and then an electron-dense nucleoid, which was identified as the DNA-containing domain. Immunofluorescence studies showed that polyprotein pp220 is localized in the viral factories. At the electron microscopic level, antibodies to pp220 labeled all identifiable forms of the virus from the precursor viral membranes onward, thus indicating an early role of the polyprotein pp220 in ASFV assembly. The subviral localization of the polyprotein products, examined on purified virions, was found to be the core shell. In addition, quantitative studies showed that the polyprotein products are present in equimolar amounts in the virus particle and account for about one-fourth of its total protein content. Taken together, these results suggest that polyprotein pp220 may function as an internal protein scaffold which would mediate the interaction between the nucleoid and the outer layers similarly to the matrix proteins of other viruses. MATERIALS AND METHODS Cells and viruses. Vero cells (ATCC CCL81) were cultured in Dulbecco's modified Eagle's medium supplemented with 10% newborn calf serum, which was reduced to 2% during viral infection. Swine alveolar macrophages, obtained by bronchoalveolar lavage, were maintained and infected in Dulbecco's modified Eagle's medium with 10% newborn calf serum. ASFV strain BA71V, adapted to grow in Vero cells, has been previously described (14). Highly purified ASFV was obtained by Percoll equilibrium centrifugation (7). Antibodies. Monoclonal antibody 18H.H7, against proteins pp220 and p150, as well as rabbit polyclonal anti-p150, anti-p37/p14, and anti-p34 sera, which also recognize the precursor form pp220, have been characterized previously (28, 33). Monoclonal antibody 19B.A2, against protein p72, has been described before (28). The anti-DNA immunoglobulin M (Ac 30-10

Rhodococcus fascians is a Gram-positive bacterium that infects dicotyledonous and monocotyledonous plants, leading to an alteration in the normal growth process of the host. The disease results from the modulation of the plant hormone balances, and cytokinins are thought to play an important role in the induction of symptoms. Generally, on the aerial parts of the plants, existing meristems were found to be most sensitive to the action of R. fascians, but, depending on the infection procedure, differentiated tissues as well gave rise to shoots. Similarly, in roots not only actively dividing cells, but also cells with a high competence to divide were strongly affected by R. fascians. The observed symptoms, together with the determined hormone levels in infected plant tissue, suggest that auxins and molecules of bacterial origin are also involved in leafy gall formation. The complexity of symptom development is furthermore illustrated by the necessary and continuous presence of the bacteria for symptom persistence. Indeed, elimination of the bacteria from a leafy gall results in the further development of the multiple embryonic buds of which it consists. This interesting characteristic offers novel biotechnological applications: a leafy gall can be used for germplasm storage and for plant propagation. The presented procedure proves to be routinely applicable to a very wide range of plants, encompassing several recalcitrant species.

Improved plants are necessary to meet human needs. Agrobacterium-mediated transformation is the most common method used to rewire plant capabilities. For plant gene delivery, DNA constructs are assembled into binary T-DNA vectors that rely on broad host range origins for bacterial replication. Here we present pLX vectors, a set of mini binary T-DNA plasmids suitable for Type IIS restriction endonuclease- and overlap-based assembly methods. pLX vectors include replicons from compatible broad host range plasmids. Simultaneous usage of pBBR1- and RK2-based pLX vectors in a two-plasmid/one-Agrobacterium strain strategy allowed multigene delivery to plants. Adoption of pLX vectors will facilitate routine plant transformations and targeted mutagenesis, as well as complex part and circuit characterization.

Serine and threonine of many nuclear and cytoplasmic proteins are posttranslationally modified with O-linked N-acetylglucosamine (O-GlcNAc). This modification is made by O-linked N-acetylglucosamine transferases (OGTs). Genetic and biochemical data have demonstrated the existence of two OGTs of Arabidopsis thaliana, SECRET AGENT (SEC) and SPINDLY (SPY), with at least partly overlapping functions, but there is little information on their target proteins. The N terminus of the capsid protein (CP) of Plum pox virus (PPV) isolated from Nicotiana clevelandii is O-GlcNAc modified. We show here that O-GlcNAc modification of PPV CP also takes place in other plant hosts, N. benthamiana and Arabidopsis. PPV was able to infect the Arabidopsis OGT mutants sec-1, sec-2, and spy-3, but at early times of the infection, both rate of virus spread and accumulation were reduced in sec-1 and sec-2 relative to spy-3 and wild-type plants. By matrix-assisted laser desorption ionization-time of flight mass spectrometry, we determined that a 39-residue tryptic peptide from the N terminus of CP of PPV purified from the spy-3 mutant, but not sec-1 or sec-2, was O-GlcNAc modified, suggesting that SEC but not SPY modifies the capsid. While our results indicate that O-GlcNAc modification of PPV CP by SEC is not essential for infection, they show that the modification has a role(s) in the process.Dynamic modification of serine and threonine with O-linked ␤-N-acetylglucosamine (O-GlcNAcylation) is widespread among nuclear and cytoplasmic eukaryotic proteins (33). The modification is essential for viability in both plants and animals (9, 23). O-GlcNAcylation is a regulatory modification that shares many common traits with protein phosphorylation (26). In addition, it has been documented for several proteins that phosphorylation and O-GlcNAcylation can occur at the same site. While the details of how O-GlcNAcylation functions in specific pathways remain to be elucidated, some general themes have emerged. The level of O-GlcNAcylation is dynamic and is influenced by hormonal signals, stress, and metabolic status, and the modification has roles in the regulation of transcription and protein synthesis and degradation. Disturbance of cellular processes regulated by O-GlcNAcylation appears to be involved in very important pathological disorders such as Alzheimer's disease (17) and diabetes (31).The enzymes responsible for O-GlcNAc addition (O-GlcNAc transferases [OGTs]) are highly conserved in plants and animals (33). OGT enzymes have an N-terminal tetratricopeptide repeat domain and a C-terminal catalytic domain. While animals have one OGT, Arabidopsis thaliana has two: Secret Agent (SEC) and SPINDLY (SPY) (9, 29). Genetic experiments have demonstrated that SEC and SPY have at least partly overlapping functions (9), but so far, there is very little information on their target proteins.O-GlcNAc modifications have been found in some structural proteins from different animal viruses such as the cytomegalovirus basic phosphoprotein (8), the adenovirus fi...