Analysis of CCAAT box transcription factor binding activity during early Xenopus laevis embryogenesis.

Unfertilized eggs and pre-midblastula (MBT) stage Xenopus embryos were found to contain a large pool of maternally derived CCAAT box-binding transcription factor (CBTF). DNA mobility shift experiments using embryonic extracts prepared with either low or high salt buffers suggest that Xenopus CBTF may not interact with embryonic DNA until the late blastula stage, a time point coincident with the increase in zygotic transcription. Additionally, photoaffinity-labeling experiments revealed that both pre- and post-MBT CBTF-binding activities were composed of at least three proteins having relative molecular masses of 68, 52, and 42 kDa.