Fig. 1. Human ribosomal genes are transcriptionally silent in hybrid cell lines. (A) HindIII-digested high molecular weight DNA from hybrid, human (HeLa) and mouse (A9) cells was probed with a human rDNA-specific hybridization probe (see Materials and methods). The source of the DNA is shown above each lane and molecular weight markers are shown on the left. (B) Total RNA samples (10 µg each) from HeLa, mouse A9 and A9HyTK hybrid cells were used in S1 nuclease protection assays with probes designed to detect the 5′ ends of mouse and human 47S precursor rRNAs (Mouse ETS and Human ETS, respectively) and a probe designed specifically to detect human 28S rRNA. RNA samples from untreated and TPA-treated cells are designated – and +, respectively. The human chromosome present in each of the hybrid cell lines is shown above and the probe used is shown on the left of the appropriate panel.