FIGURE 2. Nucleosome positioning in the endogenous Pgk2 gene in somatic and spermatogenic cells in the mouse.

The micrococcal nuclease (MNase) sensitivity assay was used to examine nucleosome positioning in the endogenous mouse Pgk2 gene in somatic cells in which the Pgk2 gene is terminally repressed, premeiotic spermatogonia in which the Pgk2 gene is undergoing potentiation prior to initiation of expression, and meiotic spermatocytes and postmeiotic spermatids in which the Pgk2 gene is actively expressed. Regions containing elevated amplimer levels represent the site of a nucleosome that inhibited access of the MNase to the underlying DNA, whereas regions of low amplimer levels represent more accessible sites devoid of nucleosomes. (A) Maps of the 5′ half of the Pgk2 gene provide context for each set of MNase assays. (B) Somatic spleen cells show the presence of nucleosomes (dark grey ovals) at the −1 region, which covers the Pgk2 enhancer and core promoter regions and at the +1 position within the Pgk2 coding sequence downstream of the transcription start site. (C) A mixed population of spermatogenic cells consisting primarily of Pgk2-expressing spermatocytes and spermatids shows the absence of a nucleosome at the −1 position in these cells (dashed oval), but the continued presence of a nucleosome at the +1 position. (D) A purified population of primitive type A spermatogonia in which the Pgk2 gene is not yet expressed shows the presence of nucleosomes at both the −1 and +1 positions, whereas in pachytene spermatocytes (E) the −1 nucleosome is completely missing, and in round spermatids (F), the −1 nucleosome presence is reduced (light grey dashed oval), while the +1 nucleosome persists in all of these cell types.