Comparison of regulation of synthesis and utilization of 45S ribosomal precursor RNA in diploid and heteroploid human cells in response to valine deprivation

Abstract

Diploid human fibroblasts respond similarly to heteroploid, malignant cells (HeLa) in decreasing both the synthesis and the utilization of 45S ribosomal precursor RNA when cells are blocked in growth by valine deprivation. During valine deprivation the diploid cells, like HeLa cells, decrease 45S r-pre-RNA synthesis to approximately half the rate in active growth, but utilize only about one-fourth of this RNA for ribosome manufacture. The diploid cells also continue synthesis, and partial utilization, of 45S r-pre-RNA for some time after protein synthesis is completely blocked with cycloheximide, as has previously been observed with HeLa cells. Thus the synthesis of ribosomal precursor RNA appears to be under non-stringent control in diploid, normal, as well as heteroploid, malignant human cells. A significant element in the regulation of ribosome synthesis in both types of cells is turnover of unused 45S r-pre-RNA.