Ribosomal S27a Coding Sequences Upstream of Ubiquitin Coding Sequences in the Genome of a Pestivirus

Molecular characterization of cytopathogenic (cp) bovine viral diarrhea virus (BVDV) strain CP Rit, a temperature-sensitive strain widely used for vaccination, revealed that the viral genomic RNA is about 15.2 kb long, which is about 2.9 kb longer than the one of noncytopathogenic (noncp) BVDV strains. Molecular cloning and nucleotide sequencing of parts of the genome resulted in the identification of a duplication of the genomic region encoding nonstructural proteins NS3, NS4A, and part of NS4B. In addition, a nonviral sequence was found directly upstream of the second copy of the NS3 gene. The 3′ part of this inserted sequence encodes an N-terminally truncated ubiquitin monomer. This is remarkable since all described cp BVDV strains with ubiquitin coding sequences contain at least one complete ubiquitin monomer. The 5′ region of the nonviral sequence did not show any homology to cellular sequences identified thus far in cp BVDV strains. Databank searches revealed that this second cellular insertion encodes part of ribosomal protein S27a. Further analyses included molecular cloning and nucleotide sequencing of the cellular recombination partner. Sequence comparisons strongly suggest that the S27a and the ubiquitin coding sequences found in the genome of CP Rit were both derived from a bovine mRNA encoding a hybrid protein with the structure NH₂-ubiquitin-S27a-COOH. Polyprotein processing in the genomic region encoding the N-terminal part of NS4B, the two cellular insertions, and NS3 was studied by a transient-expression assay. The respective analyses showed that the S27a-derived polypeptide, together with the truncated ubiquitin, served as processing signal to yield NS3, whereas the truncated ubiquitin alone was not capable of mediating the cleavage. Since the expression of NS3 is strictly correlated with the cp phenotype of BVDV, the altered genome organization leading to expression of NS3 most probably represents the genetic basis of cytopathogenicity of CP Rit.