Turnover of rat liver 28 S ribosomal RNA: nicking as the initial step of degradation.
The large ribosomal subunits of rat liver contain small amount of minor RNA components which migrate through polyacrylamide gels between the two major rRNAs, and some 28 S rRNA molecules contain hidden nicks which can be revealed by heat treatment. The minor RNA components were isolated, labeled in vitro with 125I, and hybridized with nucleolar DNA from rat liver. The hybridization-competition experiments with 28 S RNA established that the minor RNA components and 28 S rRNA have a common DNA origin. When RNA components were isolated from rats that had received [14C]orotate and [3H]orotate, 1 week and 16 h, respectively, before sacrifice, the 14C : 3H ratio was the highest in the minor components, followed by the nicked RNA, and the intact 28 S rRNA showed the lowest ratio. We also found that partial hepatectomy decreases and the actinomycin C-treatment increases the amount of the minor RNA components as well as the nicked 28 S RNA. From these observations, we conclude that the initial step of degradation of 28 S rRNA in vivo is the nick formation and that the nicked RNA subsequently gives rise to the minor components.