In vivo occupancy of histone gene proximal promoter elements reflects gene copy number‐dependent titratable transactivation factors and cross‐species compatibility of regulatory sequences

To assess systematically the structural and functional aspects of histone gene transcription within a chromosomal context, we stably integrated an extensive set of human histone H4 gene constructs into mouse C127 cells. Levels of expression were determined by S₁ nuclease protection assays for multiple mouse monoclonal cell lines containing these human H4 genes. For each cell line, we quantitated the number of integrated human H4 genes by Southern blot analysis. The results indicate that the expression of the human H4 gene is in part copy number dependent at low gene dosages. However, the level of expression varies among different cell lines containing similar numbers of copies of the same H4 gene construct. This result suggests that position‐dependent chromosomal integration effects contribute to H4 gene transcription, consistent with the roles of long‐range gene organization and nuclear architecture in gene regulation. At high copy number, the level of human H4 gene expression per copy decreased, and endogenous mouse H4 mRNA levels were also reduced. Furthermore, in vivo occupancy at the human H4 gene immediate 5′ regulatory elements, as defined by genomic fingerprinting, showed copy number‐dependent protein/DNA interactions. Hence, human and mouse H4 genes compete for titratable transcription factors in a cellular environment. Taken together, these results indicate cross‐species compatibility and suggest limited representation in vivo of the factors involved in regulating histone H4 gene transcription.