Chromatin remodeling during mammalian spermatogenesis: role of testis specific histone variants and transition proteins.

The structure of chromatin undergoes extensive alteration during mammalian spermatogenesis. Several testis specific histone subtypes are synthesised and replace their somatic counterparts during pre-meiotic, meiotic and post-meiotic stages of germ cell differentiation. Early work from our laboratory showed that pachytene spermatocyte nuclei as well as nucleosome core particle are more accessible to DNasel than the interphase liver nuclei. The higher order structure of chromatin in pachytene spermatocytes is also loosely packed due to the poor DNA and chromatin condensing property of the testis specific linker histone H1t. A careful analysis of the amino acid sequence of histone H1t revealed the absence of the DNA condensing domain containing SPKK/TPKK motifs in the C-terminus of the histone H1t. The spermiogenesis process following the meiotic division is characterised by extensive remodeling of chromatin. Transition proteins, TP1 and TP2, unique to mammalian spermatogenesis play an important role in this spermiogenesis process. We have shown that TP1 is a DNA melting protein while TP2 is a DNA condensing protein. We have delineated the molecular anatomy of TP2 including the presence of two novel zinc finger modules, which are essential for the recognition of CpG islands in the genome. TP2 is also phosphorylated by sperm specific protein kinase A and the phosphorylation/dephosphorylation cycle plays an important role in the chromatin condensation process.