DNA-dependent protein kinase specifically represses promoter-directed transcription initiation by RNA polymerase I.

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Abstract
DNA-dependent protein kinase (DNA-PK) is a nuclear enzyme that phosphorylates several transcription factors, but its cellular function has not been elucidated. Here I show that DNA-PK strongly inhibits promoter-directed transcription initiation by Xenopus RNA polymerase I in vitro. The repression is due to protein phosphorylation, since it is relieved by 6-dimethylaminopurine, an inhibitor of protein kinases. DNA-PK inhibits transcription from both linear and circular templates, but the repression is more efficient on linear templates. DNA-PK has no effect on promoter-directed transcription by RNA polymerases II and III. Partial fractionation of the in vitro transcription system shows that a protein fraction containing transcription factor Rib1, the Xenopus equivalent of human SL1, mediates the repression of transcription by DNA-PK. The present data suggest a role for DNA-PK in down-regulating ribosomal gene transcription.