Role of nonhistone chromosomal proteins in the regulation of histone gene expression

Histone gene expression was studied during the cell cycle of continuously dividing HeLa S ₃ cells and following stimulation of confluent monolayers of WI 38 human diploid fibroblasts to proliferate. The presence of histone messenger RNA (mRNA) sequences was assayed by hybridization to a ³H labeled single stranded DNA complementary to histone mRNA's. In HeLa S ₃ cells, histone mRNA sequences were found in the nucleus and associated with polyribosomes during S phase but not during G ₁. Transcripts of S phase chromatin contained histone mRNA sequences but those of G ₁ chromatin did not. Similarly, in WI 38 cells association of histone mRNA sequences with polyribosomes and transcription of histone mRNA sequences from chromatin parallel DNA replication. Together these results suggest that the regulation of histone gene expression resides, at least in part, at the transcriptional level. Chromatin reconstitution studies provide evidence that nonhistone chromosomal proteins play a key role in activation of histone gene transcription during the period of the cell cycle when DNA is replicated. Phosphate groups associated with the S phase nonhistone chromosomal proteins appear to be functionally involved in the control of histone gene readout. Although WI 38 human diploid fibroblasts transformed by SV40 exhibit morphological and biochemical modifications characteristic of neoplastic cells, transcription of histone mRNA sequences remains unaltered.