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.