Two liver-enriched trans-acting factors support the tissue-specific basal transcription from the rat tyrosine aminotransferase promoter.

The rat tyrosine aminotransferase gene (TAT) is a glucocorticoid-inducible gene, specifically expressed in liver. Using gel retardation assays, we have shown that its promoter (nt + 1 to -350; TAT.35) binds a combination of both ubiquitous and liver-specific trans-acting factors. Cis-acting sequences spanning: (i) nt -65 to -85 bound NF-Y, an ubiquitous "AACCAAT" box binding factor; (ii) nt -157 to -171 bound a liver-enriched member of the NF1 gene family [NF1Liver (NF1L hereafter)]; (iii) nt -266 to -281 bound the liver specific factor HNF1; and (iv) nt -283 to -288 bound ubiquitous "CCAAT" box binding factor(s). Moreover, the TAT gene promoter was able to drive liver-specific basal transcription, even in an in vitro assay using TAT-expressing (liver) vs non-expressing (spleen) crude nuclear extracts (NEs). Competition studies in transcription with both unmutated and mutated ds-oligonucleotides (ds-oligos) demonstrated that NF1L and HNF1 supported approx. 60 and 25% of the basal transcriptional activity sustained by TAT.35 in the liver, respectively. Neither of these oligos affected the very low level of transcription sustained by spleen NEs. This suggests a minor role for HNF1 in liver-specific basal TAT gene expression, consistent with previous observations with dedifferentiated C2 hepatoma cells (which does not express HNF1) [Deschatrette and Weiss. Biochimie 56 (1974) 1603-1611 and Cereghini et al. EMBO Jl9 (1990) 2257-2263]. Competition studies in liver-specific in vitro transcription with ds-oligo -265/-290 yielded a 90% inhibition, suggesting either that sequences spanning nt -283 to -288 sequester "CCAAT-box" binding factor(s) that may be relevant elsewhere for TAT promoter function (e.g. NF-Y which interacts with nt -65 to -85), or that such a factor interacts functionally with HNF1.