Asymmetric deletion of the junction between the short unique region and the inverted repeat does not affect viral growth in culture and vaccine-induced immunity against Marek's disease.

To construct an effective recombinant Marek's disease virus type 1 (MDV1), we localized a stable insertion site for expression of the Escherichia coli lacZ gene near or within the short inverted repeats of MDV1 strain K554 DNA. A stable recombinant MDV1 was obtained by deleting the junction region between the short unique sequence (Us) and the internal short inverted repeat (IRs). The recombinant MDV1 replicated in cultured cells as well as the parental viral DNA. Antibodies against both MDV1 antigen and beta-galactosidase encoded by the lacZ gene were detected in the sera of chickens immunized with the virus, and persisted for at least 16 weeks. Moreover, the recombinant virus conferred protection upon chickens against a challenge with virulent MDV1. These results demonstrated that the Us-IRs junction region is an effective site for the insertion of foreign genes from which to construct a polyvalent live vaccine for poultry. Analysis of the Us-IRs junction region which was deleted from the parental MDV1 indicated that there is a tandem direct repeat of a 220-bp exists within the short internal and terminal inverted repeats of avirulent MDV1 K554 strain DNA. The 220-bp sequence was well conserved among DNAs from various strains. The number of the repeat units may differ between the IRs and TRs or among various MDV1 strain DNAs.