In the present study, we report the characterization of a 212-amino-acid polypeptide encoded by a splicing variant of the Marek's disease virus Eco-Q gene (Meq). This protein, referred to as Meq-sp, contains the N-terminal 100 amino acids of Meq, which include part of Meq's DNA binding/dimerization domain, but lacks the transactivation domain of Meq. Thus, Meq-sp was examined for its ability to bind to DNA and act as a transactivator. Results indicated that while Meq and Meq-sp could both bind to the AP-1 binding site, the 110 C-terminal amino acid residues of Meq-sp lacked the ability to function as a transactivator when fused to the GAL4 (1-147) DNA binding motif. To investigate whether Meq-sp can interact with Meq or with c-jun, protein-protein and protein-DNA interactions in vitro were examined. Results showed that Meq-sp can associate with both Meq and c-jun and bind to the AP-1 site with a higher affinity as a heterodimer with c-jun. These results suggest that Meq-sp could compete with Meq for heterodimer formation with c-jun and dimer binding to DNA and possibly act as a transdominant negative regulator of Meq activity in vivo.