Fluctuations in cyclin E levels are required for multiple rounds of endocycle S phase in Drosophila.
The precise cell-cycle alternation of S phase and mitosis is controlled by alternating competence of nuclei to respond to S-phase-inducing factors [1]. Nuclei acquire competence to replicate at the low point in cyclin-dependent kinase (Cdk) activities that follows mitotic destruction of cyclins. The elevation of Cdk activity late in G1 is thought to drive cells into S phase and to block replicated DNA from re-acquiring replication competence [2]. Whereas mitosis is normally required to eliminate the cyclins prior to another cycle of replication, experimental elimination of Cdk activity in G2 can restore competence to replicate [3-6]. Here, we examine the roles of Cdks in the endocycies of Drosophila [7]. In these cycles, rounds of discrete S phases without intervening mitoses result in polyteny. Cyclins A and B are lost in cells as they enter endocycles [8,9], and pulses of Cyclin E expression drive endocycle S phases [10-12]. To address whether oscillations of Cyclin E expression are required for endocycles, we expressed Cyclin E continuously in Drosophila salivary glands. Growth of the cells was severely inhibited, and a period of DNA replication was induced but further replication was inhibited. This replication inhibition could be overcome by the kinase inhibitor 6-dimethylaminopurine (6-DMAP), but not by expression of subunits of the transcription factor E2F. These results indicate that endocycle S phases require oscillations in Cdk activity, but, in contrast to oscillations in mitotic cells, these occur independently of mitosis.
Follette
P J
PJ
Department of Biochemistry and Biophysics, University of California, San Francisco 94143-0448, USA.
Duronio
R J
RJ
O'Farrell
P H
PH
D000818:Animals; D019927:Cyclin E / Q000378:metabolism*; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D004261:DNA Replication; D004330:Drosophila / Q000166:cytology / Q000378:metabolism*; D016196:S Phase; D012469:Salivary Glands / Q000378:metabolism
D019927:Cyclin E; D018844:Cyclin-Dependent Kinases