MODELING M-PHASE CONTROL IN XENOPUS OOCYTE EXTRACTS - THE SURVEILLANCE MECHANISM FOR UNREPLICATED DNA

Alternating phases of DNA synthesis and mitosis, during the first 12 c ell divisions of frog embryos, are driven by autonomous cytoplasmic os cillations of M-phase promoting factor (MPF). Cell-free extracts of fr og eggs provide a convenient preparation for studying the molecular ma chinery that generates MPF oscillations and the surveillance mechanism that normally prevents entry into mitosis until chromosomal DNA is fu lly replicated. Early experiments suggested that unreplicated DNA bloc ks MPF activity by inducing phosphorylation of a crucial tyrosine resi due, but recent evidence implicates a stoichiometric inhibitor (an MPF binding protein) as the 'braking' agent. Using a realistic mathematic al model of the mitotic control system in frog egg extracts, we sugges t that both tyrosine phosphorylation and a stoichiometric inhibitors a re involved in the block of MPF by unreplicated DNA. Both pathways ope rate by raising the cyclin threshold for MPF activation. As a bonus, i n the process of analyzing these experiments, we obtain more direct an d reliable estimates of the rate constants in the model. (C) 1998 Else vier Science B.V. All rights reserved.