Positive regulator of viral late genes transcription. Responsible for the transition from middle to late gene expression. Activates the Plys, PI, PP and Pmom late promoters thereby allowing the expression of viral endolysin and structural genes. Activates Pmom promoter by unwinding the DNA, thus realigning the promoter elements and recruiting the RNAP. Binding magnesium induces a conformational change that allows DNA-binding. Homodimer. Expressed in the intermediate phase of the viral replicative cycle. The helix-turn-helix (HTH) motif is involved in DNA-binding. The leucine zipper (bZIP) domain is involved in dimerization. Belongs to the c/mor transcriptional regulatory family.