The non-essential UL50 gene of avian infectious laryngotracheitis virus encodes a functional dUTPase which is not a virulence factor

The DNA sequence of the infectious laryngotracheitis virus (ILTV) UL50, UL5 1 and UL52 gene homologues was determined. Although the deduced UL50 protei n lacks the first of five conserved domains of the corresponding proteins o f mammalian alphaherpesviruses, the ILTV gene product was also shown to pos sess dUTPase activity. The generation of UL50-negative ILTV mutants was fac ilitated by recombination plasmids encoding green fluorescent protein (GFP) , and expression constructs of predicted transactivator proteins of ILTV (a lpha TIF, ICP4) were successfully used to increase the infectivity of viral genomic DNA. A GFP-expressing UL50-deletion mutant of ILTV showed reduced cell-to-cell spread in vitro, and was attenuated in vivo. A similar deletio n mutant without the foreign gene, however, propagated like wild-type ILTV in cell culture and was pathogenic in chickens. We conclude that the viral dUTPase is not required for efficient replication of ILTV in the respirator y tract of infected animals. The replication defect of the GFP-expressing I LTV recombinant is most likely caused by toxic effects of the reporter gene product, since spontaneously occurring inactivation mutants exhibited wild -type-like growth.