The POU domain: a large conserved region in the mammalian pit-1, oct-1, oct-2, and Caenorhabditis elegans unc-86 gene products.

The POU domain1 (pronounced 'pow') is a highly charged 155–162-amino-acid (aa) region of sequence similarity contained within three mammalian transcription factors. Pit-1 (ref. 2), Oct-1 (ref. 3) and Oct-2 (ref. 4), and the product of the nematode gene unc-86 (ref. 5) which is involved in determining neural cell lineage. This domain consists of two subdomains, a C-terminal homoeo domain and an N-terminal POL -specific region separated by a short nonconserved linker; the sequence relationship shows that the POU homoeo domains form a distinct POU-related family. In the ubiquitous and lymphoid-specific octamer-motif binding proteins Oct-1 and Oct-2, the POU domain is sufficient for sequence-specific DNA binding3,4. Homoeobox domains contain a helix-turn-helix DNA-binding motif6,7, first identified in bacterial repressers8. The helix-turn-helix region of the POU domain is important for DNA binding3,9 and, in other classes of homoeo-containing proteins, the entire homoeo domain is sufficient for DNA binding10–12; thus the new POU-specific region could be involved in other functions such as protein–protein interactions. Nevertheless, we show here that in fact the POU domain is a novel bipartite DNA-binding structure in which the POU homoeo and POU-specific regions form two subdomains that are both required for DNA binding but are held together by a flexible linker.