The tyrosine phosphatase cdc25 selectively inhibits transcription of the Xenopus oocyte-type tRNAtyrC gene.

The Xenopus tyrosine tRNAtyrC (TyrC) genes are developmentally regulated. These multicopy genes are expressed in early oocytes and inactivated as oocytes reach maturity. As shown here, this developmental regulation can be reproduced in vitro in extracts of early and late stage oocytes: the TyrC gene is transcribed in early oocyte extracts but is virtually inactive in mature oocyte extracts. The inability to transcribe the TyrC gene is not due to the lack of functional pol III transcriptional components, since the somatic-type TyrD gene is fully active in mature oocyte extracts. Instead, the loss of TyrC transcription appears to be due to a change in the template specificity of transcription factor TFIIIC: addition of TFIIIC from immature extracts restores TyrC transcription in mature extracts. In mixtures of immature and mature extracts, the transcriptional activity of the TyrC gene is reduced. The presence of sodium vanadate, an inhibitor of tyrosine phosphatases, increases the level of TyrC transcription in the extract mixtures. Also, cdc25 phosphatase treatment of immature extracts causes a decrease in TyrC transcription which is reversed by addition of exogenous TFIIIC. These findings indicate that changes in phosphorylation state alters the template specificity of TFIIIC leading to the selective inactivation of oocyte type TyrC genes.