Abstract
During lactation, specialized cells in the mammary gland produce milk to nourish the young. Milk protein genes are controlled by distal enhancers activating expression several hundred-fold during lactation. However, the role of promoter elements is not understood. We addressed this issue using the Csn2 gene, which accounts for 10% of mRNA in mammary tissue. We identified STAT5 and other mammary transcription factors binding to three distal candidate enhancers and a cytokine-response promoter element. While deletion of the enhancers or the introduction of an inactivating mutation in a single promoter element had a marginable effect, their combined loss led to a 99.99% reduction of Csn2 expression. Our findings reveal the essential role of a promoter element in the exceptional activation of a milk protein gene and highlight the importance of analyzing regulatory elements in their native genomic context to fully understand the multifaceted functions of enhancer clusters and promoters.