5'-flanking and 5'-proximal exon regions of the two Xenopus albumin genes. Deletion analysis of constitutive promoter function.

The 5'-flanking regions and the first two exons of the 68kd and 74kd albumin genes of Xenopus laevis reveal extensive sequence homology between the two in the exon part, in the 5'-flanking region up to position -400 as well as in the first intron. Sequence comparisons of the Xenopus genes with either the albumin genes of the chicken and mammals or the mammalian alpha-fetoprotein genes reveals no homology in the 5'-flanking region but some conserved features in the first exon. The analysis of the chromatin structure demonstrates a DNase I hypersensitive region in the promoter of the 68kd albumin gene specific for hepatocytes that express the albumin gene. Deletion analysis of albumin-CAT fusion genes indicates that a 69 base-pair fragment extending from -50 to +19 of the 68 kd albumin gene is sufficient for constitutive transcription in microinjected Xenopus oocytes. The addition of 5'-flanking sequences did not change the transcriptional activity. This is consistent with the sequence data that revealed no other promoter element in this region other than the TATA box. The absence of a CCAAT box distinguishes the Xenopus albumin genes from the mammalian albumin genes but is in agreement with the promoter structure of the alpha-fetoprotein genes.