The regulation of herpes simplex virus immediate early gene (IE) expression is governed by viral and cellular proteins that assemble a multiprotein transcription enhancer complex. The analysis of the protein components and the biochemical interactions provides a model for RNAPII directed transcription and identifies critical elements of the HSV IE gene regulatory mechanism. One such component, the cellular C1 factor, is a large and complex family of polypeptides which functions to direct the assembly of the enhancer complex as well as to mediate the transcriptional activation potential of some of the associated polypeptides. An unusual characteristic of the C1 factor is the autocatalytic processing that generates the C1 family of polypeptides. Isolation of a number of cellular proteins that specifically bind the reiterated processing sites indicates that processing may serve to regulate protein-protein interactions and thus the functions of the C1 factor.