During chronic metabolic acidosis, increased expression of renal glutaminase (GA) results from selective stabilization of the GA mRNA. This response is mediated by a direct repeat of an 8-base AU-sequence that binds -crystallin and functions as a pH-response element (pH-RE). A tetracycline-responsive promoter system was developed in LLC-PK₁-F⁺ cells to perform pulse-chase analysis of the turnover of a chimeric -globin (G) mRNA that contains 960-bp of the 3'-UTR of GA mRNA including the pH-RE. The G-GA mRNA exhibits a 14-fold increase in half-life when the LLC-PK₁-F⁺ cells are transferred to acidic medium. RNase H cleavage and Northern analysis of the 3'-ends established that rapid deadenylation occurred concomitant with the rapid decay of the G-GA mRNA in cells grown in normal medium. Stabilization of the G-GA mRNA in acidic medium is associated with a pronounced decrease in the rate of deadenylation. Mutation of the pH-RE within the G-GA mRNA blocked the pH-responsive stabilization, but not the rapid decay, whereas insertion of only a 29-bp segment containing the pH-RE was sufficient to produce both a rapid decay and a pH-responsive stabilization. Various kidney cells express multiple isoforms of AUF1, an AU-binding protein that enhances mRNA turnover. RNA gel shift assays demonstrated that the recombinant p40 isoform of AUF1 binds to the pH-RE with high affinity and specificity. Thus, AUF1 may mediate the rapid turnover of the GA mRNA, whereas increased binding of -crystallin during acidosis may inhibit degradation and result in selective stabilization.