Control of RNA content of developing human placenta.

Human placentae obtained early in pregnancy or at full term were examined for RNA content per cell, RNA polymerase types and activities, and chromatin template availability. The RNA:DNA ratio fell from 0.7 at 15 to 20 weeks to 0.4 at 40 weeks of pregnancy. Since RNase activities were similar at both times, the reduction in RNA content was attributed not to increased degradation, but to reduced synthesis. At both stages of pregnancy, about 55 to 60 per cent of the RNA polymerase activity in isolated placental nuclei was accounted for by RNA polymerase II, as judged by suppression of activity with alpha-amanitin and by separation of the extracted polymerases on DEAE-Sephadex. The relative roles of changes in polymerase activity and template availability were measured in nuclei from 20- and 40-week placentae. Nuclei showed 20 per cent greater polymerase activity in full-term than in early placentae, but the template availability of isolated chromatin for transcription by RNA polymerase II was 70 per cent less at full term. We conclude that the reduced amount of RNA per cell in the full-term placenta is due to reduced template availability that more than offsets the slight increase in polymerase activity.