DNA ligase gene disruptions can depress viral growth and replication in poxvirus-infected cells

Poxvirus-encoded DNA ligases are assumed to play a role in viral DNA replication; however mutational inactivation of vaccinia ligase has not been reported to affect viral growth rates in culture. This communication re-examines this surprising aspect of poxviral biology using both Shope fibroma virus (SFV) and vaccinia virus. SFV and vaccinia ligase deficiencies create essentially identical phenotypes. In particular, ligase-deficient SFV strains are mildly UV sensitive and etoposide resistant, phenotypes previously shown to characterize ligase-deficient vaccinia strains. Moreover, we find that ligase mutations can inhibit the growth of both SFV and vaccinia virus in vitro. The poor growth observed in the absence of a viral ligase is correlated with a two- to tenfold reduction in viral and extragenomic DNA synthesis. This phenotype is host dependent. No differences in viral growth or DNA yield were seen when vaccinia strains were cultured on rabbit (SIRC) cells, but ligase deficiencies reduced growth and DNA yields when vaccinia was plated on BSC-40 cells or SFV on SIRC cells. Despite these replicative defects, mutational inactivation of SFV ligase produced no detectable increase in the number of viral DNA breaks and had no effect on virus-catalyzed extragenomic DNA recombination or UV repair. We conclude that poxviral ligases do play a role in viral DNA replication, but the replicative defect is obscured in some cell lines.