Nuclear ligation of RNA polynucleotide kinase products

RNA polynucleotide kinase has been shown to transfer [γ 32 P] from ATP to 5‐OH termini of endogenous nuclear RNA. The products of this reaction have been isolated in RNA larger than 125 after in vitro incubation of mouse L cell nuclei. About 20%–30% of these 5′‐OH kinase products are polyadenylated. A sizeable fraction of the [γ 32 P] label from ATP is also found in internal phosphodiester bonds after 30‐minute nuclear incubation in vitro. The possibility of substantial [ 32 P] recycling via the α position of nucleoside triphosphate was ruled out because: (1) 2mM nucleoside triphosphates in the incubation medium, (2) limited nearestneighbor distribution 3′ and 5′ to the phosphodiester bond compared with that from [α 32 P] UTP, (3) different nearest‐neighbor distribution for RNA molecules > 12S and 12‐3S, (4) relative insensitivity of the [γ 32 P] incorporation to α‐amanitin as compared with total RNA synthesis, (5) internal [ 32 P] appearance in RNA > 12S in less than five minutes of incubation, and (6) < 0.03% to 0.6% of the total [ 32 P] in the α position of nucleoside triphosphates after 30 minutes of incubation. The [γ 32 P] incorporation was dependent on high ATP concentration and was insensitive to competition by inorganic phosphate. These results are consistent with the levels of 5′ RNA polynucleotide kinase activity in L cell nuclei and suggest the presence of an RNA ligase that can utilize the termini generated by the 5′‐OH RNA kinase in a ligation reaction.