The absence of a human-specific ribosomal DNA transcription factor leads to nucleolar dominance in mouse greater than human hybrid cells.

The basis for nucleolar dominance in mouse-human cell hybrids which contained all of the mouse chromosomes but an incomplete set of human chromosomes (M greater than H) was examined at the molecular level. S1 mapping data showed that these cells had the expected levels of steady-state rRNA transcribed from mouse ribosomal gene (rDNA) transcription units but undetectable levels of rRNA derived from the human rDNA transcription templates that are also present. RNA polymerase I-dependent, cell-free transcription extracts were made from three hybrid lines and were found to be capable of transcribing cloned rDNA templates of mouse but not human origin. Partially purified human factors required for rDNA transcription in vitro were added to the M greater than H extracts. One fraction with almost no RNA polymerase I activity conferred on these hybrid cell extracts the ability to transcribe a human rDNA template. These rescue experiments suggested that this required human-specific rDNA transcription factor(s) was effectively absent from the lines we examined and could account for nucleolar dominance in M greater than H hybrid cells.