Regulated Transport of Ribosomal Subunits from Regenerating Rat Liver Nuclei in a Cell-free System1

The regulation of ribosome formation in eukaryotic cells still remains an intractable problem despite the extensive knowledge available concerning the details of the nucleolar synthesis and processing of the rRNA. Within the nucleolus, which is the site of ribosome formation (2, 15, 20), a 45 S precursor is methylated and combined with ribosomal proteins during transcription (33); the 45 S precursor RNA is then processed to 32 S and 20 S preribosomal species which are subsequently cleaved to yield the 28 S and 18 S species. During this processing further ribosomal proteins are added, so that the 28 S and 18 S RNA species are eventually transported to the cytoplasm in the form of 60 S and 40 S ribosomaJ subunits (31). Approximately one-half of each 45 S precursor molecule is lost during the nonconservative processing (16). The 60 S ribosomal subunit also contains a 5 S and a 7 S species of rRNA (1, 19). The mechanism regulating the rate of flow of equal numbers of the 2 ribosomal subunits to the cytoplasm is of particular importance since ribosome forma tion precedes and must be integrated with DNA synthesis and cell division. Previous in vivo studies with the rat liver system suggest (23-25) that, in addition to transcriptional control, ribosome formation is controlled at the level of nuclear processing and nucleocytoplasmic transport. Similar conclusions have been drawn from the lymphocyte system (5). Accordingly, it appears that much of the potential rRNA is ordinarily degraded in the nucleus of resting cells but is conserved in ribosomal subunits, which are transported to the cytoplasm when the cells are stimulated to proliferate or are treated with the appropriate anabolic hormones. The earlier report (26) of the release in homogenates of detectable amounts of RNA