Single-stranded deoxyribonucleic acid nuclease induced by African swine fever virus and associated to the virion.

Infection of Vero cells with African swine fever (ASF) virus resulted in a marked increase of DNase active on single-stranded DNA (ss-DNase). No increase was observed for double-stranded DNA-specific nuclease activity. In contrast to uninfected cell ss-DNase, which has a pH optimum at pH range 8.5-9, virus-induced ss-DNase is most active at pH 7. Differences in sensitivity to several ions and other modifications of the reaction mixture and considerable difference in reaction kinetics suggest that the increase in nuclease activity is due to a new virus-induced enzyme. This is strengthened by the fact that anti-ASF virus antiserum inhibits the activity of ss-DNase from infected cells but not from uninfected cells. Exclusion chromatography of the digests shows that virus-specific ss-DNase is exclusively or predominantly an endonuclease. The increase in nuclease activity of infected cells is proportional to the multiplicity of infection. Virus-specific ss-DNase is synthesized at late times after infection and its synthesis is dependent on viral DNA replication since it is not induced when infected cells are treated with cytosine arabinoside. Most of ss-DNase activity in infected cells is associated to an insoluble cytoplasmic fraction, presumably virosomes. The enzyme can also be detected in partially stripped purified virions which hydrolyze 6.9 ng DNA per microgram viral protein.