Assessment of the base sequence homology between the two subtypes of equine herpesvirus 1

Abstract

The magnitude of the genetic relatedness of the 2 antigenic subtypes of equine herpesvirus 1 (EHV-1) was determined by DNA-DNA reassociation kinetics. Denatured, labeled viral DNA from one EHV-1 subtype was allowed to reassociate in the presence or absence of the unlabeled heterologous viral DNA. The initial rate of reassociation of either labeled viral DNA was increased by the presence of the heterologous viral DNA to an extent indicating 10-20% homology between the 2 EHV-1 genomes. Similar estimates of the amount of homology between the genomes of the 2 EHV-1 subtypes were obtained by determining the maximum fraction of labeled viral DNA that could be made resistant to S1 nuclease by hybridization with a large molar excess of the unlabeled, heterologous viral DNA. Analysis of the thermal stability of the subtype 1-subtype 2 heteroduplex DNA indicated .apprx. 30% base pair mismatching within the hybrid DNA molecules. Cross-hybridization of 32P-labeled virion DNA to nitrocellulose blots of restriction endonuclease cleavage fragments of each EHV-1 subtype DNA indicated that the observed homology between the 2 viruses was nonuniformly distributed within the viral genome. No homology could be detected between the DNA of either EHV-1 subtype and that of a strain of equine cytomegalovirus (EHV-2). Evidently, the 2 biotypes of EHV-1 have arisen by divergent evolution from a common progenitor herpesvirus.