Abstract
RNA polymerase III (Pol III) is responsible for the production of small noncoding RNA species, including tRNAs and 5S rRNA. Pol III-dependent transcription is generally enhanced in transformed cells and tumors, but the underlying mechanisms remain not well-understood. It has been demonstrated that the BRF1 subunit of TFIIIB is essential for the accurate initiation of Pol III-dependent transcription. However, it is not known whether BRF1 undergoes ubiquitin modification and whether BRF1 ubiquitination regulates Pol III-dependent transcription. Here, we show that RNF12, a RING domain-containing ubiquitin E3 ligase, physically interacts with BRF1. Via direct interaction, RNF12 catalyzes Lys27- and Lys33-linked polyubiquitination of BRF1. Furthermore, RNF12 is able to negatively regulate Pol III-dependent transcription and cell proliferation via BRF1. These findings uncover a novel mechanism for the regulation of BRF1 and reveal RNF12 as an important regulator of Pol III-dependent transcription.
Highlights
RNA polymerase III (Pol III) is responsible for the production of small noncoding RNA species, including tRNAs and 5S rRNA
These findings suggest that enhanced Pol III– dependent transcription allows cancer cells to meet their high demands for protein synthesis, but it is actively involved in tumorigenesis
RNF12 failed to induce polyubiquitination of BRF1 in the presence of Ub48K or Ub63K (Fig. 4C). These results indicate that the polyubiquitin chains attached to BRF1 catalyzed by RNF12 are not linked via either Lys48 or Lys63 of ubiquitin
Summary
To investigate whether BRF1 undergoes ubiquitin modification, we performed the ubiquitination assay with WT ubiquitin or mutant ubiquitin (Ub–KO, all lysine residues replaced by arginine residues). Knockdown of RNF12 greatly increased, whereas ectopic expression of RNF12 strongly decreased the levels of pre-tRNALeu, tRNATyr, and 5S rRNA (Fig. 5, A and B), indicating that RNF12 inhibits Pol III– dependent transcription. Mutant BRF1 [521– 677], lacking RNF12-binding ability, failed to reverse the effect of RNF12 on pre-tRNALeu, tRNATyr, and 5S rRNA levels (Fig. 5B) These data suggest that RNF12 negatively regulates Pol III– dependent transcription through BRF1. The results showed that overexpression of RNF12 decreased, whereas knockdown of RNF12 increased the binding of BRF1 to the promoters of 5S rRNA and tRNALeu (Fig. 5, G and H) These findings indicate that RNF12 may inhibit Pol III– dependent transcription via decreasing the binding of BRF1 to target gene promoters
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