Murine teratocarcinoma stem cells, unlike most other cell types, do not express major histocompatibility antigens. The steady-state levels of fi-microglobulin and H-2 mRNA from F9-derived teratocarcinoma stem and differentiated cells were examined by blot hybridization using cloned DNA probes specific for these mRNAs. No H-2-or 32-microglobulin-specific RNA was detected in F9 teratocarcinoma stem cells (clone 12-1); thus, F9 teratocarcinoma stem cells (clone 12-1) contain no more than 1/ 10 the H-2 and P2-microglobulin mRNAs of the differentiated daughter cells (clone 12-la). We suggest that this regulation of major histocompatibility antigen expression is due to transcriptional control of the major histocompatibility antigen genes, H-2 and 32-microglobulin. The transcriptional regulation of these genes is accompanied by a change in their DNase I sensitivity. Normally, transcriptionally inactive genes are DNase I resistant, while active genes are DNase I sensitive. In contrast, the silent major histocompatibility antigen genes of teratocarcinoma stem cells are more DNase I sensitive than the active genes of the differentiated cells.The regulation of expression of the major histocompatibility antigens has intrigued immunologists and developmental biologists for many years. These antigens are detected on the surface ofnearly all types ofsomatic cells. Murine teratocarcinoma stem cells, like cells of the early mouse embryo (1-4), do not express these antigens (5-9). However, differentiated cell lines derived from the teratocarcinoma stem cell, like other somatic cells, do express these antigens (5-7). Until recently, the control of the expression of the histocompatibility genes could be analyzed only by serological and biochemical analysis of the histocompatibility protein antigens. With the availability ofcloned molecular DNA probes, we can now examine both the mRNA transcripts and the state of the genomic DNA in teratocarcinoma-derived stem cells and differentiated cells. We report here blot hybridization analysis of RNA extracted from the F9-derived stem and differentiated cell lines, 12-1 and 12-la, respectively, using cloned 832-microglobulin and H-2 probes. In addition, we have examined the DNase I sensitivity of the H-2 and 82-microglobulin genes to determine whether specific murine genes within stem cell chromatin are, like total stem cell chromatin (10), DNase I hypersensitive regardless oftheir transcriptional state.
MATERIALS AND METHODSCells. 12-1 teratocarcinoma stem cells were isolated (11) after transfection ofthymidine kinase (Tk)-deficient F9 cells (12) Isolation of Nuclei. All manipulations after collection ofcells were at 4°C. Cells were scraped from glass bottles and washed twice in phosphate-buffered saline and once in reticulocyte standard (RS) buffer (0.01 M Tris-HCl, pH 7.4/0.01 M NaCl/ 3 mM MgCl2) and nuclei were isolated by suspension in RS buffer/0.5% Nonidet P40 (British Drug House, Poole, England) essentially as described by Weintraub and Groudine (21). Nuclei were washed twic...