Reassessment of histone gene expression during cell cycle in human cells by using homologous H4 histone cDNA.

Abstract

The representation of H4 histone mRNA sequences in RNA isolated from G1 and S phase HeLa [human cervical cancer] cells was assessed by use of a homologous H4 histone cDNA. S phase cells were obtained by double thymidine block, and G1 cells were obtained by double thymidine block or mitotic selective detachment. Nuclear and cytoplasmic RNA from S phase cells hybridized with H4 histone cDNA as did nuclear and cytoplasmic RNA from G1 cells synchronized by double thymidine block. In contrast, significant levels of hybridization were not observed between H4 histone cDNA and nuclear, polysomal, or postpolysomal cytoplasmic RNA of G1 cells synchronized by mitotic selective detachment. Double thymidine block yields a G1 cell population containing 20-25% S phase cells whereas the G1 population obtained by mitotic detachment contains < 0.1% S phase cells. The ability of H4 histone cDNA to hybridize with the RNA from G1 cells obtained after release from double thymidine block can be explained by the presence of S phase cells in such a G1 population.sbd.an artifact of the synchronization procedure. These results are interpreted to be consistent with the presence of H4 histone mRNA sequences during the S but not G1 phase of the cell cycle in continuously dividing HeLa S3 cells.