Despite its key role in potassium homeostasis, transcriptional control of the H ϩ -K ϩ -ATPase ␣2-subunit (HK␣2) gene in the collecting duct remains poorly characterized. cAMP increases H ϩ -K ϩ -ATPase activity in the collecting duct, but its role in activating HK␣2 transcription has not been explored. Previously, we demonstrated that the proximal 177 bp of the HK␣2 promoter confers basal collecting duct-selective expression. This region contains several potential cAMP/Ca 2ϩ -responsive elements (CRE). Accordingly, we examined the participation of CRE-binding protein (CREB) in HK␣2 transcriptional control in murine inner medullary collecting duct (mIMCD)-3 cells. Forskolin and vasopressin induced HK␣2 mRNA levels, and CREB overexpression stimulated the activity of HK␣2 promoter-luciferase constructs. Serial deletion analysis revealed that CREB inducibility was retained in a construct containing the proximal 100 bp of the HK␣2 promoter. In contrast, expression of a dominant negative inhibitor (A-CREB) resulted in 60% lower HK␣2 promoter-luciferase activity, suggesting that constitutive CREB participates in basal HK␣2 transcriptional activity. A constitutively active CREB mutant (CREB-VP16) strongly induced HK␣2 promoter-luciferase activity, whereas overexpression of CREBdLZ-VP16, which lacks the CREB DNA-binding domain, abolished this activation. In vitro DNase I footprinting and gel shift/supershift analysis of the proximal promoter with recombinant glutathione S-transferase (GST)-CREB-1 and mIMCD-3 cell nuclear extracts revealed sequence-specific DNA-CREB-1 complexes at Ϫ86/ Ϫ60. Mutation at three CRE-like sequences within this region abolished CREB-1 DNA-binding activity and abrogated CREB-VP16 trans-activation of the HK␣2 promoter. In contrast, mutation of the neighboring Ϫ104/Ϫ94  element did not alter CREB-VP16 transactivation of the HK␣2 promoter. Thus CREB-1, binding to one or more CRE-like elements in the Ϫ86/Ϫ60 region, trans-activates the HK␣2 gene and may represent an important link between rapid and delayed effects of cAMP on HK␣2 activity. transcription; promoter; cAMP; potassium POTASSIUM AND ACID-BASE HOMEOSTASIS must be strictly maintained and restored for the proper functioning of all eukaryotic cells. Epithelial cells of the kidney and colon play a critical role in adjusting K ϩ , Na ϩ , and acid elimination to accommodate changes in dietary intake. Physiological and molecular biological studies in animals have pointed to the H ϩ -K ϩ -ATPase ␣ 2 -subunit gene (HK␣ 2 , also termed colonic H ϩ -K ϩ -ATPase), which is principally expressed in the distal colon and renal collecting duct, as a key participant in the control of body K ϩ homeostasis (14,29,30,35). For example, mice with null mutations in this gene experience profound hypokalemia during dietary K ϩ (18) and Na ϩ restriction (31). HK␣ 2 may also play a role in HCO 3 Ϫ absorption by the kidney (20, 21) and the distal colon (18), as well as increased ammonium secretion in the inner medullary collecting duct (IMCD) during chronic hypokal...