THE POSITION AND LENGTH OF THE STEROID-DEPENDENT HYPERSENSITIVE REGION IN THE MOUSE MAMMARY-TUMOR VIRUS LONG TERMINAL REPEAT ARE INVARIANT DESPITE MULTIPLE NUCLEOSOME-B FRAMES

Stimulation of the mouse mammary tumor virus with steroids results in the generation of a DNase I-hypersensitive region (HSR) spanning the h ormone responsive element (HRE) in the long terminal repeat. Restricti on enzymes were used to characterize the accessibility of various site s within the HSR of mouse mammary tumor virus long terminal repeat-rep orter constructions in four different cell lines. The glucocorticoid-d ependent HSR was found to span minimally 187 bases, a stretch of DNA l onger than that associated with histones in the core particle. Althoug h the 5'-most receptor binding site within the HRE is downstream of -1 90, hypersensitive sites were found further upstream to at least -295. The relationship in the accessibility between pairs of sites in the v icinity of the HSR was further examined in one cell line by a two-enzy me restriction access assay. In the uninduced state, the accessibiliti es at these sites were found to be independent of each other. In contr ast, when stimulated with hormone, the accessibilities at these sites were observed to become linked. That is, once a distinct promoter was activated, all of the sites within the HSR of that molecule became acc essible. The HSR formed along an invariant stretch of DNA sequence des pite the multiplicity of nucleosome frames in the nucleosome B region, where the I-IRE is located, The results indicate that the macroscopic length of the HSR does not arise from core length-remodeling events i n molecules containing Nuc-B in alternative positions.