Figure 1.

Regulation of TBP and TBP2 during oogenesis and the early stages of embryogenesis in vertebrates. Continuous line, frog; dashed line, mouse; red, TBP2; blue, TBP; green, general transcription. Stages are represented at the top by light to dark shading, and at the bottom by schematic representations. At most stages of oogenesis only TBP2 is expressed, which promotes oocyte-specific transcription during these stages. Upon meiotic maturation, TBP2 is actively degraded following global repression of transcription in maturing oocytes (as has been demonstrated in frogs). After fertilization and during the early stages of embryogenesis, TBP expression reaches the maximum levels that are needed to start zygotic transcription. In frogs zygotic transcription is largely delayed until the mid blastula and this process is regulated by late translation of maternal stores of tbp mRNA. In frog (and zebrafish) there are low levels of TBP2 during early stages of embryogenesis, whereas in mice no TBP2 has been detected during embryogenesis. Global zygotic transcription initiation is delayed in both frog and mouse, albeit to different developmental stages, and trace levels of zygotic transcription have been detected in both species before global genome activation. The figure has been generated by summarizing experiments described in [4-7,9,10].