Modular structure of the beta-globin and the TK promoters.

Three regions important for transcription, the ATA box, the middle and the distal element, have been identified in the 5'-flanking region of both the rabbit beta-globin and herpes simplex virus thymidine kinase (TK) gene. To determine whether these elements are functionally equivalent, we constructed mosaic promoters containing all combinations of the three regions from both promoters and joined them to the beta-globin transcription unit. In an enhancer-dependent transient expression assay the beta-globin, the TK and all mosaic promoters retaining the beta-globin ATA box were about equally active, however, mosaic promoters with the TK ATA box were 4- to 10-fold less active. We conclude that homologous elements are, in principle, exchangeable and suggest an explanation why certain combinations of elements function poorly.