REGULATORY ELEMENTS IN THE FIRST INTRON OF THE RAT FATTY-ACID SYNTHASE GENE

Sequence elements have been identified within the 1.2 kb-long first in tron of the fatty acid synthase (FAS) gene that mediate both positive and negative effects on transcription. The negative regulatory element , when positioned downstream of either the FAS or simian virus 40 prom oter, down-regulates the expression of a coupled reporter gene in an o rientation-dependent manner. Sequences mediating this effect have been mapped, by deletion mutagenesis, to two regions approximately within nucleotides +405 to +768 and +924 to +1083. Both regions contain seque nce elements that are strongly protected from DNase I digestion by nuc lear extracts prepared from liver, but not by those prepared from sple en. The results of run-on assays performed with nuclei derived from ti ssues that express FAS at either high or low levels indicate that the different rates of transcription of the endogenous FAS gene result fro m differences in the extent of initiation, so it is unlikely that the negative effect is caused by transcriptional pausing in the first intr on. The positive element maps to nt +292 to +297 and corresponds to an authentic binding site for upstream stimulatory factor (USF). This US E-binding element can up-regulate transcription from a heterologous pr omoter in a position- and orientation-independent manner. However, in the context of the entire FAS first intron, the effect of the USF-bind ing site is masked unless the effect of the negative elements is ablat ed by mutagenesis. These results suggest that the dominant negative el ement of the first intron may play a role in determining the tissue-sp ecific expression of the FAS gene.