Developmental Regulation of MCM Replication Factors in Xenopus Laevis

At the midblastula transition (MBT) during Xenopus laevis development, zygotic transcription begins [1], and the rapid, early cleavage cycles are replaced by cell-division cycles that lengthen and acquire G (gap) phases [2] and checkpoints [3-5]. This cell-cycle remodeling may result from either a loss of maternal products, the transcription of zygotic genes, or the replacement of maternal proteins by zygotic gene products. We have identified an example of the third possibility: distinct maternal and zygotic genes encoding a member of the minichromosome maintenance (MCM) protein family. The mcm genes were identified in yeast by mutations that blocked replication of artificial chromosomes or perturbed the G1/S transition in the cell cycle [6,7]. In Xenopus eggs, the MCM2-MCM7 proteins assemble as multimeric complexes at chromosomal origins of replication [8-14]. The sequential, cell-cycle-dependent assembly of the origin replication complex (ORC), CDC6 protein and the MCM complex at origins of replication ensures that DNA replicates only once per cell cycle [15,16]. The periodic association of the MCM complex with chromatin may be regulated via phosphorylation by cyclin-dependent kinases (Cdks) [11]. We have cloned the first example of a developmentally regulated mcm gene, zygotic mcm6 (zmcm6), expressed only after gastrulation when the cell cycle is remodeled. The zMCM6 protein assembles into MCM complexes and differs from maternal MCM6 (mMCM6) in having a carboxy-terminal extension and a consensus cyclin-Cdk phosphorylation site. There may also be maternal-zygotic pairs of other MCMs. These data suggest that MCMs are critical for cell-cycle remodeling during early Xenopus development.