The mouse albumin promoter and a distal upstream site are simultaneously DNase I hypersensitive in liver chromatin and bind similar liver-abundant factors in vitro.

Abstract

In this paper we characterize the chromatin structure and nuclear proteins associated with different transcriptional states of the mouse serum albumin gene. We found the albumin gene to be transcribed in liver at rates 1000-fold or greater than in other tissues tested. We discovered seven DNase I hypersensitive sites encompassing the albumin gene only in liver chromatin, with strong hypersensitivity at the promoter and the enhancer, which is over 10 kb upstream. Using a gel retardation assay, we found a liver nuclear protein, or set of proteins, which binds specifically to DNA of a liver-specific hypersensitive site that maps 3.5 kb upstream, between the promoter and enhancer. Footprinting, heat insensitivity, and binding competition experiments indicate that the protein(s) have characteristics similar to a heat-stable, liver-abundant protein that binds to the albumin promoter and other enhancer and promoter sequences. Finally, we asked whether the liver-specific factors that cause DNase I hypersensitivity in vivo are present concurrently at the various sites in chromatin. We devised a simple new method to reveal that in liver, individual albumin genes are hypersensitive simultaneously at the promoter, the enhancer, and the -3.5-kb site. Thus, transcriptionally active albumin genes appear to contain tissue-abundant factors that are present at three widely spaced points in chromatin, yet at the same point in time. Similar factors binding simultaneously to at least two of these sites could create a specific structure in chromatin required for high-level albumin gene transcription.