Changes in the number of binding sites for ribonucleic acid polymerase in chromatin of WI-38 fibroblasts stimulated to proliferate.

1. When WI-38 human diploid fibroblasts form confluent monolayers, DNA synthesis and cell division almost completely cease. A change of medium causes these density-inhibited cells to proliferate and within 1h after the application of the stimulus there is an increase in template activity of the chromatin isolated from stimulated cells. 2. The number of binding sites for Escherichia coli RNA polymerase was determined on chromatin from WI-38 cells by two different methods, i.e. incorporation of [(3)H]UTP into RNA in the absence of reinitiation, and incorporation of [gamma-(32)P]GTP into chain termini. 3. Both methods indicate that the capacity of chromatin to bind E. coli RNA polymerase is increased in WI-38 cells stimulated to proliferate. 4. The increase in the number of binding sites for E. coli RNA polymerase parallels the increase in chromatin template activity and suggests that the latter reflects an increase in the number of initiation sites, rather than an increase in the rate of transcription.