The structural organization of sperm chromatin.

The packaging of the male haploid genome within the differentiating spermatid nucleus is facilitated by small basic nuclear proteins called protamines. Although the majority of the DNA in human sperm chromatin is bound by these proteins, a small percentage retains a nucleosomal-like component. These histone-enriched regions may possess enhanced nuclease sensitivity and have been postulated to designate certain genes involved in early embryogenesis. We have shown previously that the chromatin domain containing the two human protamines PRM1 and PRM2 and the transition protein TNP2 forms a DNase I-sensitive conformation in pachytene spermatocytes, a requisite event prior to the haploid expression of its members in round spermatids (Kramer, J. A, McCarrey, J., Djakiew, D., and Krawetz, S. A. (1998) Development 125, 4749-4755). Interestingly, this configuration persists in mature spermatozoa subsequent to the transcriptional silencing of the locus. It was therefore postulated that the retained, enhanced DNase I-sensitive conformation of the PRM1-->PRM2-->TNP2 domain in human sperm may be preferentially histone-enriched. To address this tenet, we examined the chromatin structure of the human PRM1--> PRM2--> TNP2 domain using a PCR-based assay. The results show that this retained, enhanced DNase I sensitive domain reflects an enrichment of histones at discrete regions across the locus. In addition, a similar examination of other genes and repetitive sequences suggests the non-random distribution of histones and protamines within the sperm nucleus. A discussion of these results and their functional significance is presented.