hnRNP U inhibits carboxy-terminal domain phosphorylation by TFIIH and represses RNA polymerase II elongation.
This study describes a potential new function of hnRNP U as an RNA polymerase (Pol II) elongation inhibitor. We demonstrated that a subfraction of human hnRNP U is associated with the Pol II holoenzyme in vivo and as such recruited to the promoter as part of the preinitiation complex. hnRNP U, however, appears to dissociate from the Pol II complex at the early stage of transcription and is therefore absent from the elongating Pol II complex. When tested in the human immunodeficiency virus type 1 transcription system, hnRNP U inhibits elongation rather than initiation of transcription by Pol II. This inhibition requires the carboxy-terminal domain (CTD) of Pol II. We showed that hnRNP U can bind TFIIH in vivo under certain conditions and inhibit TFIIH-mediated CTD phosphorylation in vitro. We find that the middle domain of hnRNP U is sufficient to mediate its Pol II association and its inhibition of TFIIH-mediated phosphorylation and Pol II elongation. The abilities of hnRNP U to inhibit TFIIH-mediated CTD phosphorylation and its Pol II association are necessary for hnRNP U to mediate the repression of Pol II elongation. Based on these observations, we suggest that a subfraction of hnRNP U, as a component of the Pol II holoenzyme, may downregulate TFIIH-mediated CTD phosphorylation in the basal transcription machinery and repress Pol II elongation. With such functions, hnRNP U might provide one of the mechanisms by which the CTD is maintained in an unphosphorylated state in the Pol II holoenzyme.
Kim
M K
MK
Genetics and Biochemistry Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD 20892, USA. kimm@gwgate.nhlbi.nih.gov
Nikodem
V M
VM
D034722:Heterogeneous-Nuclear Ribonucleoprotein U; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D019384:Nucleic Acid Synthesis Inhibitors / Q000378:metabolism; D010766:Phosphorylation; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D011494:Protein Kinases / Q000378:metabolism; D017434:Protein Structure, Tertiary; D012319:RNA Polymerase II / Q000037:antagonists & inhibitors / Q000378:metabolism*; D012097:Repressor Proteins / Q000378:metabolism*; D012261:Ribonucleoproteins / Q000378:metabolism*; D051758:Transcription Factor TFIIH; D014157:Transcription Factors / Q000378:metabolism*; D019473:Transcription Factors, TFII; D014158:Transcription, Genetic
D034722:Heterogeneous-Nuclear Ribonucleoprotein U; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D019384:Nucleic Acid Synthesis Inhibitors; D012097:Repressor Proteins; D012261:Ribonucleoproteins; D014157:Transcription Factors; D019473:Transcription Factors, TFII; D051758:Transcription Factor TFIIH; D011494:Protein Kinases; C060137:carboxy-terminal domain kinase; D012319:RNA Polymerase II