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 r eplication; however mutational inactivation of vaccinia ligase has not been reported to affect viral growth rates in culture. This communica tion 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 res istant, phenotypes previously shown to characterize ligase-deficient v accinia strains. Moreover, we find that ligase mutations can inhibit t he growth of both SFV and vaccinia virus in vitro. The poor growth obs erved in the absence of a viral ligase is correlated with a two- to te nfold reduction in viral and extragenomic DNA synthesis. This phenotyp e is host dependent. No differences in viral growth or DNA yield were seen when vaccinia strains were cultured on rabbit (SIRC) cells, but l igase deficiencies reduced growth and DNA yields when vaccinia was pla ted on BSC-40 cells or SFV on SIRC cells. Despite these replicative de fects, mutational inactivation of SFV ligase produced no detectable in crease in the number of viral DNA breaks and had no effect on virus-ca talyzed extragenomic DNA recombination or UV repair. We conclude that poxviral ligases do play a role in viral DNA replication, but the repl icative defect is obscured in some cell lines. (C) 1998 Elsevier Scien ce B.V. All rights reserved.