Zebrafish cyclin E regulation during early embryogenesis.

Cyclin E cDNA, cloned from a zebrafish embryonic cDNA library, was used for analysis of cyclin E regulation during early embryogenesis. During the rapid cell cycles of the early cleavage stage, which lacks a G1 phase, the cyclin E mRNA, protein, and associated H1 kinase activity were found to be constitutive, in contrast to their reported cyclic behavior during the cycle of cultured mammalian cells. These results suggest an additional role for cyclin E during early embryogenesis, in addition to its established role during the G1/S transition in somatic cells. These results support previous identification of cyclin E in early cleaving Drosophila and Xenopus embryos, and provide for the first time the direct demonstration of constitutive cyclin E activity throughout the M/S cycles of the embryonic cleavage stage. Cyclin E mRNA was reduced during epiboly (approximately 6-8 hr postfertilization, HPF), concomitantly with a marked reduction in cell division rates. In contrast, the cyclin E protein and cyclin E-CDK complexes remained constant throughout the first 24 hr, implying that the cyclin E protein is regulated post translationally and is not immediately affected by the levels of the corresponding mRNA. However, the cyclin E-CDK complexes present in 26 somite embryos (22 HPF) did not exhibit histone H(i) kinase activity. This discrepancy between high levels of cyclin E-CDK complexes and low enzymatic activity may be explained by the presence of putative cyclin E-CDK inhibitory mechanism. Here we show that multiple levels of regulation of the cyclin E mRNA, protein, and associated kinase activity are present during the first 24 hr of zebrafish embryonic development.