Regulatory DNA sequence elements were functionally identified in the 5'-flanking region of a gene, PD1, which encodes a porcine classical transplantation antigen. Both a positive regulatory element and a novel negative regulatory DNA element were mapped within 1.1 kilobases upstream of exon 1. The negative regulatory element reduced the activity of both the homologous PD1 promoter and a heterologous simian virus 40 promoter. In vivo competition experiments indicated that the functions of the PD1 positive and negative regulatory elements are mediated by distinct cellular trans-acting factors. The PD1 positive regulatory element interacted with cellular factors in common with those binding to the simian virus 40 enhancer. Finally, the negative regulatory element required the presence of a positive regulatory element to function. This interaction between positive and negative regulatory elements represents a novel mechanism for regulating gene expression.

Ehrlich, R., Maguire, J. E., & Singer, D. S. (1988). Identification of negative and positive regulatory elements associated with a class I major histocompatibility complex gene. Molecular and Cellular Biology, 8(2), 695–703.