FIG. 2.

Identification of a B-cell-specific DNase I HS at the hCD79b/GH locus. A. Two DNase I HS are detected within and immediately 5′ of the CD79b gene; HSB1 is B cell specific. Chromatin samples from the B-cell lines were subjected to DNase I HS mapping. For the left panel, chromatin preparations from six human B-cell lines representing various developmental stages (697, REH, 25A, 9068, 1484, and 1621) were analyzed. The relative level of CD79b mRNA in each cell line, as determined from Northern blot analysis (Fig. 1 and data not shown), is indicated above the corresponding lanes (strong, ++; intermediate, +; low, +/−). The first lane contains a DNA size marker (M). The analyzed EcoRI fragment is 12.5 kb (marked by a black dot). Sub-bands generated by graded DNase I digestions (times of 0 s, 10 s, and 1 min) are indicated by the horizontal arrows to the right of the autoradiograph (HSB1 and HSa). The band at 8 kb was present irrespective of DNase I digestion (time 0) and was considered to be a cross-hybridizing band of unknown origin. For the right panel, DNase I HS mapping of chromatin from a human choriocarcinoma cell line (JEG3) and the erythroleukemia cell line (K562) were compared to a pre-B-cell line (Nalm6). HSB1 was only detectable in Nalm6 cells, whereas HSa was present in all three lines. The diagram below the blots summarizes the strategy and results of the HS mapping studies depicted in the blots. The relevant EcoRI restriction enzyme sites, the probe (Prb1; Table 2; filled box below the CD79b gene), fragment sizes, and positions of HSB1 and HSa, as well as pituitary cell-specific HSI and HSII for reference, are shown within the 12.5-kb EcoRI fragment. B. Three DNase I HS are detected in the remote 5′-flanking region of CD79b; HSB2 is B cell specific. Chromatin mapping and labeling of the autoradiograph are as described for panel A. The 23.1-kb EcoRI fragment (marked by a black dot) studied with probe Prb2 is located immediately 5′ to the fragments depicted in panel A. The DNase I sub-bands generated from this fragment are HSV, HSIII, and HSB2 (positions indicated at the right of each autoradiograph). The left panel shows DNase I mapping of the human B-cell lines; positions of HSV, HSIII, and HSB2 are indicated at the right. The right panel shows DNase I mapping of two nonlymphoid lines (JEG3 and K562) and a B-cell line (CA46). HSB2 was formed in the B-cell line but not in JEG3 or K562. The band below the 23.1-kb master band in panel B is a nonspecific, cross-hybridizing band. The diagram at the bottom summarizes the mapping experimental scheme and results as described in the legend to panel A. The hybridization probe (Prb2) is indicated by the filled rectangle. C. Localization of B-cell-specific HS in the hCD79b/GH locus. Arrows (open) above the map indicate B-cell-specific HS and the HSa identified in the current report. Arrows (closed) summarize HS identified in prior studies. HSI and HSII are specific to pituitary chromatin, HSIV is specific to placental chromatin, and HSIII and HSV are constitutive.