Regulation of human small nuclear RNA gene transcription by the tumor suppressor protein p53

Activation of the tumor suppressor protein p53 or loss of the Cockayne syndrome complementation group B (CSB) protein induces fragile site formation at RNA polymerase II-transcribed U1 and U2 snRNA gene loci and at RNA polymerase III-transcribed 5S rRNA gene loci. Yu et al. (2000) hypothesized that p53 interferes with transcription elongation functions of CSB, resulting in accumulation of stalled RNA polymerase and impaired chromatin condensation at these gene loci. However, a role for p53 and CSB in transcription of these genes has not been investigated.;In this study I show that both p53 and CSB are involved in human U1 snRNA and 5S rRNA gene transcription. I found that p53 represses U1 snRNA and 5S rRNA gene transcription by RNA polymerases II and III, respectively. p53 also represses U6 snRNA gene transcription by RNA polymerase III. Both DNA binding competent and defective forms of p53 revealed similar levels of snRNA promoter occupancy during transcription repression, suggesting that sequence-specific DNA binding by p53 is not essential for repression of snRNA gene transcription. I further demonstrated that CSB plays a positive role in snRNA gene transcription by both polymerases II and III and a negative role in transcription of those other classes of RNA polymerase III-transcribed genes that contain an intragenic arrangement of promoter elements.;The functional interplay between p53 and CSB in snRNA gene transcription was also investigated. Firstly, removing CSB from cell extracts modulates p53 transcription activity in vitro. CSB immunodepletion potentiates the inhibitory effect of p53 on U1 snRNA gene transcription, but does not affect p53-mediated repression of U6 snRNA gene transcription. Interestingly, at low amounts p53 activates and at higher amounts represses 5S rRNA gene transcription when transcription is performed with CSB depleted extracts. Secondly, CSB association with snRNA gene promoters was diminished after UV light treatment concomitant with increased p53 promoter association. As CSB was described as an elongation factor for RNA polymerase II, p53 may affect elongation by interfering with CSB promoter association. Thirdly, transient transfection of p53 results in snRNA gene transcription repression concomitant with accumulation of covalently modified forms of RNA polymerase III. These forms of RNA polymerase III are more enriched in CSB cells, suggesting that p53 and CSB have opposing roles in post-translational modifications of RNA polymerase III. I speculate that p53 represses elongation by RNA polymerase III by facilitating post-translational modifications of the polymerase. Together, these results suggest that p53 may modulate CSB transcriptional activity and support the hypothesis that fragile sites at U1 and U2 snRNA and 5S rRNA gene loci may be caused the inhibitory effect of p53 on CSB-mediated elongation by RNA polymerases.