On the mechanism of nucleolar dominance in mouse-human somatic cell hybrids

The mechanism of nucleolar dominance was studied in two lines of mouse-human somatic hybrids. Both lines had preferentially lost human chromosomes but had retained significant amounts of both mouse and human ribosomal genes (genes coding for the 18S, 5.8S, and 28S RNAs of ribosomes). However, the human ribosomal genes were repressed, and only mouse ribosomal genes were expressed. Soluble transcription extracts from the hybrids were able to initiate RNA synthesis accurately on a cloned mouse ribosomal gene but were unable to initiate accurately on a human ribosomal gene. This suggests that nucleolar dominance in these hybrids is due to the loss or inactivation of the gene for a specificity factor required to recognize the human ribosomal gene promoter.