Oncogenic Marek's disease herpesvirus encodes an isoform of the conserved regulatory immediate early protein ICP27 generated by alternative promoter usage

Herpesvirus gene expression is temporally regulated, with immediate early (IE), early (E) and late (L) genes. ICP27, which is involved in post-transcriptional regulation, is the only IE gene product conserved in all herpesviruses. We show here that the ICP27 transcript of the oncogenic Marek's disease virus shares the same polyadenylation signal as the bicistronic glycoprotein K-ICP27 transcript and is regulated by alternative promoter usage, with transcription from its own promoter (pICP27) or that of gK (pgK). The pgK can generate a spliced ICP27 transcript yielding an N-terminal-deleted ICP27 isoform (ICP27 Delta N) that, like ICP27, co-localizes with the SR protein in infected cells, but with a diffuse nuclear distribution. The pICP27 includes functional responsive elements (REs) for SP1, AP1 and CREB, is essentially active during the lytic phase and leads to exclusive expression of the native form of ICP27. The alternative promoter, pgK, including active REs for GATA, P53 and CREB, preferentially generates the gK transcript during the lytic phase and the spliced ICP27 transcript (ICP27 Delta N) during the latent phase. An analysis of the DNA methylation marks of each promoter showed that pgK was systematically demethylated, whereas pICP27 was methylated during latency and demethylated during the lytic stage. Thus, MDV ICP27 gene expression is dependent on alternative promoters, the usage of which is regulated by DNA methylation, which differs between viral stages.