The 3'-nontranslated region of rat renal glutaminase mRNA contains a pH-responsive stability element

Rat kidney expresses two forms of glutaminase (GA) mRNA which probably result from the use of alternative polyadenylation signals. The two mRNAs are increased coordinately in response to metabolic acidosis via a mechanism that apparently does not involve transcriptional or translational regulation. A 956-bp fragment that contains the 3'-nontranslated sequence of the smaller GA cDNA was cloned into an expression vector (p beta G) that encodes a chimeric beta-globin growth hormone mRNA. Both the parent and the derived construct (p beta G-GA) were transfected into LLC-PK1-F+ cells. Stable transfectants express sixfold lower levels of beta G-GA mRNA than that of the parent beta G mRNA. However, only the beta G-GA mRNA is increased 2.5-fold by growth in acidic medium (pH 6.9, 10 mM HCO3-). The apparent half-life of the beta G mRNA (> 24 h) is unaffected by the pH of the growth media. In contrast, the apparent half-life of the beta G-GA mRNA is increased from 4.5 h to approximately 24 h when cells are transferred to acidic medium for 8 h. The observed pH response is not reproduced when the beta G-GA construct is stably transfected into COS-7 cells or when a beta-globin-phosphoenolpyruvate carboxykinase chimeric gene is expressed in LLC-PK1-F+ cells. Thus the 3'-nontranslated region of the GA mRNA contains a pH-responsive stability element.