DNA-DEPENDENT PROTEIN-KINASE SPECIFICALLY REPRESSES PROMOTER-DIRECTEDTRANSCRIPTION INITIATION BY RNA-POLYMERASE-I

DNA-dependent protein kinase (DNA-PK) is a nuclear enzyme that phospho rylates several transcription factors, but its cellular function has n ot been elucidated. Here I show that DNA-PK strongly inhibits promoter directed transcription initiation by Xenopus RNA polymerase I in vitr o. The repression is due to protein phosphorylation, since it is relie ved by 6-dimethylaminopurine, an inhibitor of protein kinases. DNA-PK inhibits transcription from both linear and circular templates, but th e repression is more efficient on linear templates. DNA-PK has no effe ct on promoter-directed transcription by RNA polymerases LT and Ln. Pa rtial fractionation of the in vitro transcription system shows that a protein fraction containing transcription factor Rib1, the Xenopus equ ivalent of human SL1, mediates the repression of transcription by DNA- PK. The present data suggest a role for DNA-PK in down-regulating ribo somal gene transcription.