| The Marek's disease virus (MDV) unique short (U{dollar}sb{lcub}rm s{rcub}{dollar}) region encodes three glycoproteins, glycoproteins D (gD), I (gI), and E (gE). MDV gI and gE were detected in MDV-infected duck embryo fibroblast (DEF) cells, but gD was not. The absence of a detectable level of gD expression could be explained by a defective open reading frame, inefficient antibody, inefficient transcription, or inefficient translation. When the gD, gI, and gE genes were subjected to in vitro coupled transcription and translation in the absence or presence of canine pancreatic microsomal membranes, their precursor polypeptides or glycoproteins were produced, respectively, and could be immunoprecipitated by respective antibodies. The results of Northern blot and RT-PCR analysis showed that the only gD-containing transcript is a low abundance 7.5 kb transcript that initiated two genes upstream of the gD gene, and it is unlikely to be translated into gD protein. Thus inefficient transcription is the main reason for absence of detectable gD expression in cell culture. When the gD, gI and gE proteins were transiently expressed in COS7 cells, gD expression was not as efficient as expression of gI and gE. After the N-terminal coding region of gD was replaced with N-terminal codons of gI, expression of the gI-gD hybrid was as efficient as expression of gI. Therefore, a negative regulatory element exists in the N-terminal coding region of the gD gene that may contribute to the absence of detectable gD expression in cell culture. DEF cells were infected with commonly used MDV strains. Although gI was detected in all of the infected cell cultures, gD expression was under the limit of detection. Therefore, absence of detectable gD expression in cell culture is a general phenomenon of MDV. MDV may be more closely related to varicella-zoster virus (VZV), which does not have gD homolog, than to other alphaherpesviruses. It is known that VZV gI and gE play important roles in virus infection. Thus, based on this research, expression of MDV gI and gE will be a more important focus for future study. |
