NFY OR A RELATED CCAAT BINDING-FACTOR CAN BE REPLACED BY OTHER TRANSCRIPTIONAL ACTIVATORS FOR COOPERATION WITH HNF1 IN DRIVING THE RAT ALBUMIN PROMOTER INVIVO

Like many eukaryotic genes, the rat albumin promoter contains a CCAAT consensus motif at position - 80. In transfected H4II hepatoma cells the strength of this promoter depends to a large extent on the integrity of a hepatic nuclear factor 1 (HNF1) binding site located at position - 60 and to a lesser extent on the CCAAT element. However, if the affinity for HNF1 is reduced, the CCAAT-box becomes essential for high, and tissue specific, promoter activity. We wished to determine which, among the different CCAAT binding factors co-existing in eukaryotic cells, was responsible for this co-operativity with HNF1. To this end we prepared a series of mutants of the CCAAT sequence and compared their effects on albumin promoter activity in vivo and on the binding of different CCAAT binding factors in vitro. Our results strongly suggest that a ubiquitous factor NFY (also designated CBF, ACF, CP1) interacts with this CCAAT element in vivo. We propose that during development NFY could facilitate transcription of the albumin gene in hepatocytes when the concentration of HNF1 is limiting. This co-operativity in transcriptional activation is not due to strict co-operativity in DNA binding between the two proteins and is not limited to NFY or a closely related factor, as the CCAAT-box can be replaced by AP1, SP1 or E2 target sites without significantly affecting the final activity. © 1991.