The transcription factor LFB1 (HNF1) was initially identified as a regulator of liver-specific gene expression in mammals. It interacts with the promoter element HP1, which is functionally conserved between mammals and amphibians, suggesting that a homologous factor, XLFB1, also exists in Xenopus laevis. To study the role of LFB1 in early development, we isolated two groups of cDNAs coding for this factor from a Xenopus liver cDNA library by using a rat LFB1 cDNA probe. A comparison of the primary structures of the Xenopus and mammalian proteins shows that the myosin-like dimerization helix, the POU-A-related domain, the homeodomain-related region, and the serine/threonine-rich activation domain are conserved between X. lwvis and mammals, suggesting that all these features typical for LFB1 are essential for function. Using monoclonal antibodies, we demonstrate that XLFB1 is present not only in the liver but also in the stomach, intestine, colon, and kidney. In an analysis of the expression of XLFB1 in the developing Xenopus embryo, XLFB1 transcripts appear at the gastrula stage. The XLFB1 protein can be identified in regions of the embryo in which the liver diverticulum, stomach, gut, and pronephros are localized. The early appearance of XLFB1 expression during embryogenesis suggests that the tissue-specific transcription factor XLFB1 is involved in the determination and/or differentiation of specific cell types during organogenesis.In recent years, a growing number of transcription factors that coordinate the activity of target genes and are involved in the determination of a particular cellular fate or phenotype have been defined. In some cases, the importance of these regulatory proteins and their corresponding target sequences in the DNA is supported by the observation that the transacting factors and their cis elements have been conserved throughout evolution. Concerning liver-specific gene expression, four different types of regulatory elements and transacting factors have been identified. The prototypes of these factors involved in liver-specific transcription are C/EBP, LFB1 (HNFl), HNF3, and HNF4 (reviewed in references 17 and 27).By analyzing the liver-specific expression of the Xenopus laevis albumin genes, we have identified the hepatocytespecific promoter element HP1, which has been conserved throughout evolution, as it is functional in mammalian cells (41). HP1 not only constitutes one of the most important regulatory elements for liver-specific expression of mammalian albumin genes (15,24,29,39) but also is involved in the expression of a group of genes specifically expressed in hepatocytes, such as the genes encoding a-fetoprotein, a1-antitrypsin, and a-and 3-fibrinogens (9, 14, 24). HP1 is recognized by the liver transcription factor LFB1 (14), also referred to as HNF1 or HNFla (9, 31). The cDNA encoding LFB1 has been cloned from rats (4, 7, 12), mice (26), and humans (2). The deduced amino acid sequences for these three mammalian proteins are highly conserved and revealed that LFB1 is the most ...

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LFB1 (HNF1) is a tissue-specific transcription factor found in the livers, stomachs, intestines, and kidneys of vertebrates. By analyzing the promoter of the Xenopus LFB1 gene, we identified potential autoregulation by LFB1 and regulation by HNF4, a transcription factor with a tissue distribution similar to that of LFB1.Injection of LFB1 promoter-chloramphenicol acetyltransferase constructs into Xenopus eggs revealed embryonic activation that is restricted to the region of the developing larvae expressing endogeneous LFB1. Proper embryonic activation was also observed with a rat LFB1 promoter. Deletion analysis of the Xenopus and rat promoters revealed that in both promoters embryonic activation is absolutely dependent on the presence of an element that contains CCNCTCTC as the core consensus sequence. Since this element is recognized by the maternal factor OZ-1 previously described by N. Ovsenek, A. M. Zorn, and P. A. Krieg (Development 115:649-655, 1992), we might have identified the main constituents of a hierarchy that leads via LFB1 to the activation of tissue-specific genes during embryogenesis.Tissue-specific expression of genes is achieved to a large extent by the interaction of transcription factors with cisacting elements present in the promoters and enhancers of genes that are expressed differently in different tissues. These transcription factors themselves are tissue specific. This restricted expression pattern is established sometime during embryogenesis, and we assume that it plays a key role in the differentiation processes. Molecular analysis of early vertebrate development has established a complex hierarchy of regulatory factors involved in establishing the body plan (for recent reviews, see references 12 and 15). Some of the genes encoding these factors are the homeobox genes known to be conserved between Drosophila melanogaster and mammals (for a recent review, see reference 26). There is increasing evidence that these genes form a very complex regulatory network in which specific genes influence the activities of other genes. Typically these regulatory genes are expressed in distinct areas of the embryo and may be active only transiently during embryogenesis. There is, at least in the case of D. melanogaster, clear evidence that some transcription factors are already present in fertilized eggs, and it is assumed that these maternal factors are responsible for initiating gene activation in early embryogenesis (for a recent review, see reference 16). So far no direct link between the early active transcription factors and the expression of tissue-specific transcription factors could be made. In one approach to this question, we analyzed the regulatory elements and factors involved in embryonic activation of the promoter of the liver transcription factor LFB1 (HNF1) in Xenopus laevis.

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