Factors that interact with the rat albumin promoter are present both in hepatocytes and other cell types.

The rat albumin promoter inserted in adenovirus directs transcription in human and rodent hepatoma cells and in rodent hepatocytes (Friedman et al. 1986) and Babiss et al. (1986) but not in HeLa cells or myeloma cells. The nucleotides between -43 and -156 of the RNA start site of the rat albumin gene are required for this cell-specific expression. Protein binding studies (footprints, exonuclease III stops, and gel shifts) all indicate specific interaction in the -80 to -130 region of the gene with factors present in nuclear extracts of hepatocytes and hepatomas, but also from extracts of other cells that do not express the albumin gene. To observe albumin promoter binding, a smaller amount of extract of liver cell nuclei was required compared to extracts of HeLa cell or kidney cell nuclei. In addition, the various tests of DNA-protein interaction did not give qualitatively identical results with extracts from different cells. However, it seems clear that factors are present in several cell types where albumin genes are inactive that will bind to those DNA sequences demonstrated to be necessary for cell-specific expression of this gene. These factors could either be similar but nonidentical factors or the same factors that are modified differently in different cell types.