Dominant-negative mutants reveal a role for the Cdk7 kinase at the mid-blastula transition in Drosophila embryos

The metazoan cyclin-dependent kinase Cdk7 was purified originally as part o f a biochemical activity called CAK (Cdk-activating kinase) capable of phos phorylating and activating in vitro the Cdks that promote the different cel l cycle transitions. Cdk7 is also found in the transcription factor complex TFIIH, suggesting that it participates in vivo in the control of RNA polym erase II. We have examined the physiological role of Cdk7 during the course of Drosophila development. By expressing dominant-negative forms of the ki nase, we were able to alter Cdk7 function at given developmental stages. Ex pression of Cdk7 mutants severely delayed the onset of zygotic transcriptio n in the early embryo, but did not alter the timing of the first 13 embryon ic nuclear cycles. These results implicate Cdk7 in the control of transcrip tional machinery in vivo. While cell cycle regulation is not sensitive to o ur manipulations of Cdk7 activity, it suggests that a distinct pool of CAK activity that is unaffected by expression of the cdk7(DN) mutants is presen t in these embryos.