Transcription factors binding to the mouse HTF9 housekeeping promoter differ between cell types

Themouse CpGisland HTF9harbours a bidirectional promoter shared bytwohousekeeping genesthat are arranged head-to-head. Wehavepreviously identified several protein binding-elements acrosstheCpG island, yeta short region aroundtheinitiation region was foundtobecapable ofbidirectional transcription intransient expression assays,suggesting thatthe multiple elements oftheHTF9promoter arefunctionally redundant. We havenow comparedthebinding activities innuclear extracts fromdifferent cell types. Twoprotein-binding elements ofHTF9interact with widely distributed factors. A potentially strongSpl binding site wasalsoidentified, however Splappeared tobindefficiently toits target sequencewithextracts prepared fromproliferating cultured cells, butnotfrom adult organs.On theotherhand,theCCAATbox upstream ofonegene(HTF9-A) interacted with aliverenriched factor, whereas nobinding was detected with cultured fibroblasts extracts. Consistently, deletion of theCCAATboxaffected transient expression fromthe HTF9-Apromoter inhepatocyte, butnotinfibroblast, cultures. Our results suggestthatubiquitous expression ofhousekeeping promoters results fromthe activation ofalternative elements indifferent cell types.