Summary auto-generated
This study identifies and characterizes a gene encoding the large subunit of ribonucleotide reductase (RR1) in two baculoviruses: Spodoptera exigua and S. littoralis multicapsid nucleopolyhedroviruses (SeMNPV and SpliMNPV). The researchers found that both viruses contain open reading frames encoding RR1 proteins that share 70-80% amino acid similarity with eukaryotic RR1 proteins and retain critical catalytic residues. The RR1 genes are located in different genomic positions between the two viruses: upstream of the polyhedrin gene in SeMNPV (in antisense orientation) and preceding the p74 gene in SpliMNPV. Northern blot and primer extension analyses showed that SeMNPV rr1 is expressed as an early gene with a ~2.7 kb transcript. Searches revealed RR1-like sequences in three additional baculovirus species. Phylogenetic analysis indicated that SeMNPV and SpliMNPV acquired their RR1 genes independently from eukaryotic sources rather than inheriting them from a common viral ancestor, contrasting with the monophyletic origin observed in herpesviruses.
Key findings
- Two baculovirus species (SeMNPV and SpliMNPV) encode functional ribonucleotide reductase large subunit (RR1) genes with high amino acid similarity to eukaryotic RR1 proteins
- The baculoviral RR1 genes are located at different genomic positions in the two viruses and exhibit different transcriptional start sites, despite encoding similar proteins
- SeMNPV rr1 is expressed as an early gene throughout infection with multiple transcriptional start sites that do not match previously characterized baculovirus promoter consensus sequences
- RR1-like sequences were identified in three additional baculovirus species, suggesting the gene is widespread in the family
- Phylogenetic analysis indicates SeMNPV and SpliMNPV independently acquired RR1 genes from eukaryotic sources rather than from a common ancestral virus
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Abstract
In the genomes of two baculoviruses, Spodoptera exigua and S. littoralis multicapsid nucleopolyhedroviruses (SeMNPV and SpliMNPV, respectively), an open reading frame (ORF) encoding the large subunit of ribonucleotide reductase (RR1) was identified. The predicted amino acid sequences of SeMNPV and SpliMNPV RR1 showed high homology to RR1 proteins from eukaryotes (ca. 70% and 80% similarity, respectively). The amino acid residues thought to be involved in catalytic function were conserved in the baculoviral RR1 ORFs. The RR1 ORFs in SeMNPV and SpliMNPV were located in different genomic positions. In SeMNPV, the RR1 ORF was located upstream of the polyhedrin gene, in an anti-genomic orientation. In SpliMNPV, the RR1 ORF preceded the p74 gene. By searching databanks, sequences homologous to the N terminus of RR1 were also detected upstream of the polyhedrin gene of three other baculoviruses, Mamestra brassicae multicapsid NPV, Panolis flammea multicapsid NPV and Orgyia pseudotsugata single nucleocapsid NPV. The baculovirus type species, Autographica californica multicapsid NPV, however, does not encode RR. A 2.7 kb transcript could be detected throughout infection with SeMNPV, classifying SeMNPV rr1 as an early gene. Primer extension analysis revealed several early and late start sites. None of the major start sites showed similarity to previously characterized baculoviral transcriptional start motifs. Phylogenetic analysis of prokaryotic, eukaryotic and viral RR1 proteins suggested that SeMNPV and SpliMNPV acquired the gene for RR1 from a eukaryotic source, but independently from each other.