C/EBP-like proteins binding to the functional box-alpha and box-beta of the second enhancer of hepatitis B virus

The second enhancer (enhancer II) of hepatitis B virus is functionally liver specific. Located within an open reading frame of the virus and immediately upstream of the initiation sites of viral major transcripts, enhancer II furnishes a unique model for use in investigating the structure and function of an enhancer. In this study, two functional constituents, a 23-bp box-alpha and a 12-bp box-beta, are identified as being both necessary and sufficient for enhancer II function. Examination of the box-alpha and box-beta sequences reveals a weak homology to the extended consensus for a C/EBP binding site. Gel shift and footprinting analyses indicate that multiple proteins bind to these sequences and thus are candidate transcription factors that mediate the enhancer function. One heat-resistant protein, protein a, and one heat-sensitive protein, protein b, bind to box-alpha. Protein a, which binds to box-alpha in a way indistinguishable from that seen with a recombinant C/EBP, appears not to be identical to C/EBP in that the binding of protein a requires a minimal sequence larger than the canonical C/EBP sites. Two box-beta-binding proteins, c and d, show greater affinity for the C/EBP consensus than for box-beta. However, both proteins c and d are relatively heat sensitive and display a distinct sequence preference from the recombinant C/EBP protein. Since the function of enhancer II is strictly dependent on a bipartite architecture, this system provides a unique model for studies of how the interactions of its binding proteins lead to the enhancer function.