Tissue-specific expression is conferred by a sequence from the 5' end of the rat albumin gene

We have constructed a transient expression vector containing 400 bp of rat albumin gene immediate 5'-flanking sequences inserted 5' to the bacterial enzyme chloramphenicol acetyl transferase (CAT). We have transfected various clones of rat hepatoma cells representing different states of expression of the liver phenotype with this vector (pALB-cat) and also with two control vectors containing viral promoters (pSVE-cat and pRSV-cat), and measured activity of the bacterial enzyme CAT in cellular extracts 48 h later. The albumin flanking sequences are able to direct highly efficient CAT expression, compared with the control vectors, only in cells which express their own albumin gene: the albumin-negative hepatoma cells are at least 100 times less efficient in expressing CAT after transfection with the pALB-cat plasmid than are the albumin-positive ones. An unexpected result of our study is the total inability of the rat albumin flanking sequences to direct expression in albumin-producing mouse hepatoma cells.