Genomic duplication and translocation of reactivation transactivator and bZIP-homolog genes is a conserved event in alcelaphine herpesvirus 1

Alcelaphine herpesvirus 1 (AlHV-1) is a gammaherpesvirus carried asymptomatically by wildebeest. Upon cross-species transmission, AlHV-1 induces malignant catarrhal fever (MCF), a fatal lymphoproliferative disease of ruminants, including cattle. The strain C500 has been cloned as an infectious, pathogenic bacterial artificial chromosome (BAC) that is used to study MCF. Although AlHV-1 infection can be established in cell culture, multiple passages in vitro cause a loss of virulence associated with rearrangements of the viral genome. Here, sequencing of the BAC clone showed that the long unique region (LUR) of the genome is nearly identical to that of the previously sequenced strain from which the BAC was derived, and identified the duplication and translocation of a region from within LUR, containing the entire coding sequences of ORF50-encoding reactivation transactivator Rta and A6-encoding bZIP protein genes. The duplicated region was further located to a position within the terminal repeat (TR) and its deletion resulted in lower ORF50 expression levels and reduced viral fitness. Finally, the presence of a similar but not identical duplication and translocation containing both genes was found in AlHV-1 strain WC11. These results indicate that selection pressure for enhanced viral fitness may drive the duplication of ORF50 and A6 in AlHV-1.