The co-transcriptional editing of the Newcastle disease virus (NDV) P gene has been studied by sequence analysis of cloned viral genomic RNA and mRNA. Evidence has been obtained for the specific insertion of non-templated G nucleotides, the consequence of which is the generation of three populations of P gene-derived mRNAs. The three populations encode proteins (P, V and W) which have a common N-terminal region, but which utilize three different reading frames at their C termini. Paradoxically, NDV edits its P gene mRNA by the insertion of non-templated G residues in a manner similar to Sendai and measles viruses (P-~ V editing) despite its apparent closer evolutionary relationship to the simian virus type 5, mumps and related group of viruses which edit a V genomic sequence to generate an mRNA to encode a functional P protein (V -~ P editing).

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Protein assemblies with a high degree of repetitiveness and organization are known to induce strong immune responses. For that reason they have been postulated for the design of subunit vaccines by means of protein engineering. The enzyme lumazine synthase from Brucella spp. (BLS) is highly immunogenic, presumably owing to its homodecameric arrangement and remarkable thermodynamic stability. Structural analysis has shown that it is possible to insert foreign peptides at the ten amino terminus of BLS without disrupting its general folding. These peptides would be displayed to the immune system in a highly symmetric three-dimensional array. In the present work, BLS has been used as a protein carrier of foreign peptides. We have established a modular system to produce chimeric proteins decorated with ten copies of a desired peptide as long as 27 residues and have shown that their folding and stability is similar to that of the wild-type protein. The knowledge about the mechanisms of dissociation and unfolding of BLS allowed the engineering of polyvalent chimeras displaying different predefined peptides on the same molecular scaffold. Moreover, the reassembly of mixtures of chimeras at different steps of the unfolding process was used to control the stoichiometry and spatial arrangement for the simultaneous display of different peptides on BLS. This strategy would be useful for vaccine development and other biomedical applications.

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