Switching of TATA binding proteins between oocytes and embryos. (A) In Xenopus oocytes TBP2 but not TBP is present at the protein level. Upon meiotic maturation (MM) TBP2 is degraded after global shut down of transcription, but residual levels of TBP2 persist in egg and early embryo. During early cleavages after fertilization, maternal stores of TBP mRNA are translated and by the mid-blastula transition (MBT) both TBP and residual TBP2 contribute to zygotic transcription leading to a partial switching of function between TBP and TBP2. In X. laevis TBP2 RNA is detected well into larval stages, whereas in X. tropicalis TBP2 mRNA levels decline during gastrulation (our unpublished data). (B) Mouse oocytes also express only TBP2. When zygotic transcription commences at the two cell stage of the mouse embryo, TBP2 function is replaced by translationally upregulated TBP leading to a complete switching of TATA binding factors.