Nitsch, Doris; Stewart, A. F.; Boshart, Michael; Mestril, Ruben; Weih, Falk; Schütz, Günther
Chromatin Structures of the Rat Tyrosine Aminotransferase Gene Relate to the Function of Its cis-Acting Elements.
In: Molecular and Cellular Biology, Vol. 10, No. 7: pp. 3334-3342
The relationship between DNase I-hypersensitive sites (HSs) and transcriptional enhancers of the rat tyrosine
aminotransferase (TAT) gene was examined by comparing HSs in and around the TAT gene with the activity
of the corresponding DNA sequences in transient transfection assays. In this manner, we identified two HSs as
liver-specific enhancers. Of three hepatoma cell lines examined, only one sustained TAT mRNA levels
comparable to those of liver. In this cell line, both enhancers were strongly active, and strong hypersensitivity
in chromatin over the enhancers was evident. The other two hepatoma cell lines had reduced levels of TAT
mRNA and no or altered hypersensitivity over either the enhancers or the promoter. One of these lines carried
a negative regulator of the TAT gene, the tissue specific extinguisher Tse-1. This cell line exhibited all HSs
characteristic of the strongly active gene except at the promoter; however, one enhancer was inactive even
though hypersensitive in chromatin. In a TAT-nonexpressing cell line, inactivity of both enhancers correlated
with absence of the respective HSs. We conclude that although hypersensitivity in chromatin necessarily
accompanies cell-type-specific enhancer activity, the occurrence of cell-type-specific HSs does not imply that the
underlying sequences harbor enhancers active in transient transfection assays.