Abstract
The mouse CpG island HTF9 harbours a bidirectional promoter shared by two housekeeping genes that are arranged head-to-head. We have previously identified several protein binding-elements across the CpG island, yet a short region around the initiation region was found to be capable of bidirectional transcription in transient expression assays, suggesting that the multiple elements of the HTF9 promoter are functionally redundant. We have now compared the binding activities in nuclear extracts from different cell types. Two protein-binding elements of HTF9 interact with widely distributed factors. A potentially strong Sp1 binding site was also identified, however Sp1 appeared to bind efficiently to its target sequence with extracts prepared from proliferating cultured cells, but not from adult organs. On the other hand, the CCAAT box upstream of one gene (HTF9-A) interacted with a liver-enriched factor, whereas no binding was detected with cultured fibroblasts extracts. Consistently, deletion of the CCAAT box affected transient expression from the HTF9-A promoter in hepatocyte, but not in fibroblast, cultures. Our results suggest that ubiquitous expression of housekeeping promoters results from the activation of alternative elements in different cell types.