High-resolution analysis of gene activity during the Xenopus mid-blastula transition

The Xenopus mid-blastula transition (MBT) marks the onset of large-scale zygotic transcription, as well as an increase in cell cycle length and a loss of synchronous cell divisions. Little is known about what triggers the activation of transcription or how newly expressed genes interact with each other. Here we use high-resolution expression profiling to identify three waves of gene activity: a post-fertilization wave involving polyadenylation of maternal transcripts; a broad wave of zygotic transcription detectable as early as the 7th cleavage and extending beyond the MBT at the 12th cleavage; and a shorter post-MBT wave of transcription that becomes apparent as development proceeds. Our studies have also allowed us to define a set of maternal genes whose mRNAs are deadenylated shortly after fertilization, and are likely to be degraded thereafter. Experimental analysis indicates that the polyadenylation of maternal transcripts is necessary for the establishment of proper levels of zygotic transcription at the MBT, and that genes activated in the second wave of expression, including Brachyury and Mixer, contribute to the regulation of genes expressed in the third. Together, our high-resolution time series and experimental studies have yielded a deeper understanding of the temporal organization of gene regulatory networks in the early