Phosphorylation of human general transcription factors TATA-binding protein and transcription factor IIB by DNA-dependent protein kinase - Synergistic stimulation of RNA polymerase II basal transcription in vitro

DNA-dependent protein kinase (DNA-PK) has been known to catalyze phosphorylation of a number of regulator factors involved in DNA replication and transcription such as simian virus 40 T antigen, p53, c-Myc, Sp1, and RNA polymerase II (Pol II). We examined the possibility that DNA-PK phosphorylates the general transcription factors TATA-binding protein (TBP) and transcription factor (TF) IIB. which play key roles in the formation of transcription initiation complex with Pol II. By using a highly purified preparation of DNA-PK from Raji cells, both TBP and TFIIB were shown to be phosphorylated in vitro by DNA-PK. We then investigated the effect of the phosphorylation of these factors on Pol II basal transcription, Stepwise analysis of preinitiation complex formation by electrophoretic mobility shift assay revealed that the phosphorylation of TBP and TFIIB by DNA-PK did not affect the formation of promoter (P)-TBP and P-TBP-TFIIB complexes but synergistically stimulated the formation of P-TBP-TFIIB-TFIIF-Pol II complex. Similarly, combination of the phosphorylated TBP and TFIIB, synergistically stimulated Pol II basal transcription from adenovirus major late promoter. These observations suggest that DNA-PK could positively regulate the pol II basal transcription by phosphorylating TBP and TFIIB.