Fig. 5.

The molecular events that precede transcription of the protamine domain. Before acquiring a potentiated conformation in the pachytene spermatocyte, the mouse protamine domain is marked at upstream sites B and C by Top2a and Top2b in leptotene and zygotene spermatocytes. The promoters of Prm2 and Tnp2 are marked by histone (H3K9/14) acetylation. Coincident with the potentiation of the domain, a large region immediately upstream of the Top2 cleavage sites of the gene cluster is recruited to the nuclear matrix. The promoters of all three genes and CNS1 within the domain are acetylated. A single site (site 7) of Top2a activity was detected toward the proximal end of this ≈9.6-kb MAR in pachytene spermatocytes. The genes of the protamine domain are transcribed postmeiotically in round spermatids. At this time, two MARs that flank the domain bind the nuclear matrix in conjunction with extensive histone acetylation throughout the domain and a single upstream CNS2. Significant induction of Top2a site 6 and Top2b site 7 activity were detected upstream of the domain. Top2 functions in concert with other nuclear matrix factors to mark this multigenic domain for potentiation and eventual transcription.