IDENTIFICATION OF 2 STEPS DURING XENOPUS RIBOSOMAL GENE-TRANSCRIPTIONTHAT ARE SENSITIVE TO PROTEIN-PHOSPHORYLATION

Protein kinase(s) and protein phosphatase(s) present in a Xenopus S-10 0 transcription extract strongly influence promoter-dependent transcri ption by RNA polymerase I. The protein kinase inhibitor 6-dimethyl-ami nopurine causes transcription to increase, while the protein phosphata se inhibitor okadaic acid causes transcription to decrease. Repression is also observed with inhibitor 2, and the addition of extra protein phosphatase 1 stimulates transcription, indicating that the endogenous phosphatase is a type 1 enzyme. Partial fractionation of the system, single-round transcription reactions, and kinetic experiments show tha t two different steps during ribosomal gene transcription are sensitiv e to protein phosphorylation: okadaic acid affects a step before or du ring transcription initiation, while 6-dimethylaminopurine stimulates a process ''late'' in the reaction, possibly reinitiation. The present results are a clear demonstration that transcription by RNA polymeras e I can be regulated by protein phosphorylation.