ABSTRACT
Ribosomal ribonucleic acid (rRNA) in eukaryotic cells is made up of two principal components: an 18s RNA and a 28s RNA. These two RNAs make up the bulk of intracellular RNA. The "primer hypothesis" has been proposed as one possibility for explaining gene control by "activator" RNA. This hypothesis also proposes a source for the unique activator RNAs required to activate each gene uniquely. Primer RNA could be used to produce steady-state conditions of RNA transcription, utilization and turnover of a nonchanging population of cell RNA, or to produce changes in the pattern of RNA synthesis during developmental sequences. Reutilization of degradation fragments as primer RNA segments could lead to additional controls on the development of differentiated functions. Several lines of evidence in support of the primer hypothesis have been available. They include: the presence and properties of intracellular low molecular weight RNA; RNA activating transcription; RNA transfer from cytoplasm to nucleus; and conservation of RNA segments.
