Binding of RFX2 and NF-Y to the testis-specific histone H1t promoter may be required for transcriptional activation in primary spermatocytes.

The testis-specific linker histone H1t is transcribed exclusively in pachytene spermatocytes during spermatogenesis. The H1t promoter contains two imperfect inverted repeats which together comprise the X-box motif that is known to bind the transcription factor regulatory factor X (RFX). Out of all the histone H1 family promoters this motif appears only in the H1t promoter and may contribute to H1t tissue-specific expression. We show by Western blotting, EMSA, ChIP analysis, and real-time RT-PCR that the rat H1t X-box is bound by RFX2 in vivo in spermatocytes. We demonstrate that transcription factor NF-Y binds to the CCAAT-box motif that is located downstream and adjacent to the X-box and that testis NF-Y interacts either directly or indirectly with RFX2. Furthermore, we show that both the X-box and CCAAT-box are required for promoter activity and that co-expression of RFX2 greatly enhances testis histone H1t promoter activity in the GC-1spg germinal cell line.