TRANSCRIPTIONAL HIERARCHY IN XENOPUS EMBRYOGENESIS - HNF4 A MATERNAL FACTOR INVOLVED IN THE DEVELOPMENTAL ACTIVATION OF THE GENE ENCODING THE TISSUE-SPECIFIC TRANSCRIPTION FACTOR HNF1-ALPHA (LFB1)

The tissue specific transcription factor HNF1 alpha (LFB1) expressed i n liver, kidney, stomach and gut gets transcriptionally activated in X enopus shortly after zygotic transcription starts. By microinjection i nto fertilized Xenopus eggs, a HNF1 alpha promoter fragment is activat ed in the middle part of developing larvae, reflecting the activation pattern of the endogenous HNF1 alpha gene. Mutational analysis of the HNF1 alpha promoter shows that HNF1 and HNF4 binding sites are essenti al for proper embryonic regulation. Since by injecting HNF4 mRNA into fertilized eggs the endogenous HNF1 alpha gene is activated ectopicall y and HNF4 is present as a maternal protein within an animal to vegeta l gradient in the embryo, we assume that HNF4 initiates a transcriptio nal hierarchy involved in determination of different cell fates.