In response to physiological cues, a common strategy for regulating gene expression is the modulation of RNA stability. Polyadenylation at the 3′-end of mRNAs is a major structural feature that appears to play an important role in message stability in many cases (Tharun and Parker, 1997). During development, regulated changes in length of the poly(A) + tail (PAT) of specific messages are correlated with their stability and translation (Richter, 1999). Deadenylation and 5′ decapping are rate-limiting steps in yeast mRNA decay (Muhlrad et al., 1995), while in mammals the 3′ untranslated region (UTR) and RNA binding proteins also have major effects on degradation rates (Ross, 1995). Mechanisms of RNA decay have been elucidated in bacteria (Xu and Cohen, 1995;Sarkar, 1997;Blum et al., 1999), chloroplasts (Hayes et al., 1999;Schuster et al., 1999), plant mitochondria (Gagliardi and Leaver, 1999;Kuhn et al., 2001;Lupold et al., 1999) and the eukaryotic cytoplasm (Mitchell and Tollervey, 2000). Longer PATs are generally associated with longer-lived mRNA species, and in some cases play a direct role in translational regulation as well (Rajagopalan and Malter, 1997;Sachs, 2000). In contrast to the eukaryotic cytoplasm, the degradation rate of poly(A) + mRNA is increased compared to nonadenylated mRNA in chloroplasts, plant mitochondria, and bacteria. Polyadenylated mRNAs from trypanosomes were also recently shown to degrade more rapidly in the presence of elevated UTP concentrations (Militello and Read, 2000). The unusual mitochondrial genomes and transcriptional mechanisms of trypanosomes, however, make generalizations problematic. Any potential significance to the different roles the PAT may play regarding mRNA degradation in the cytoplasm vs. organelles is unclear and, to date, animal mitochondria have not been studied in depth regarding RNA degradation. Although mRNAs in animal mitochondria are known to be polyadenylated (Beutow and Wood, 1978), to our knowledge there are no reports directly assessing the effect of polyadenylation on mitochondrial mRNA stability. We Polyadenylation of messenger RNA is known to be an important mechanism for regulating mRNA stability in a variety of systems, including bacteria, chloroplasts and plant mitochondria. By comparison, little is known about the role played by polyadenylation in animal mitochondrial gene expression. We have used embryos of the brine shrimp Artemia franciscana to test hypotheses regarding message stability and polyadenylation under conditions simulating anoxia-induced quiescence. In response to anoxia, these embryos undergo a profound and acute metabolic downregulation, characterized by a steep drop in intracellular pH (pHi) and ATP levels. Using dot blots of total mitochondrial RNA, we show that during in organello incubations both O2 deprivation and acidic pH (pH·6.4) elicit increases in half-lives of selected mitochondrial transcripts on the order of five-to tenfold or more, relative to normoxic controls at pH·7.8. Polyadenylation of these transcripts was me...