Metabolic characteristics of rat liver cytosolic 5SRNA

5SRNA were purified from the cytosol, ribosomes, nucleoli and nucleoplasm of rat liver. The 5'-terminal and 3'-terminal sequences of these 5SRNA species were shown to be the same. The time course of changes in specific activities of these 5SRNA in rat liver, 20 min to 16 h after intraperitoneal injection of [14C]orotic acid, were investigated. The specific activity of cytosolic 5SRNA increased almost linearly for 3 h, without a lag, to a high value, then decreased rapidly. The specific activity of cytosolic 5SRNA was much higher than the specific activities of 5SRNA from the other cellular components over 45 min to 6 h after intraperitoneal injection, although at 20 min the specific activity was similar to that of nucleoplasmic 5SRNA. Thus, cytosolic 5SRNA of rat liver is characterized by a high rate of turnover, which was shown to be comparable to that of heterogeneous nuclear RNA. The specific activity of 5SRNA, 45 min to 6 h after intraperitoneal injection, was as follows; cytosolic > nucleoplasmic > nucleolar > ribosomal 5SRNA. The specific activity of nucleoplasmic RNA that migrated slightly slower than nucleoplasmic 5SRNA on PAGE was much higher than that of cytosolic 5SRNA or nucleoplasmic 5SRNA, and so may be the precursor of nucleoplasmic 5SRNA. Together, these results indicate that cytosolic 5SRNA is transferred from the nuclei to the cytosol immediately after maturation of the precursor of nucleoplasmic 5SRNA without entering the nucleoplasmic pool of 5SRNA.