INOSINE EXISTS IN MESSENGER-RNA AT TISSUE-SPECIFIC LEVELS AND IS MOSTABUNDANT IN BRAIN MESSENGER-RNA

The general view that mRNA does not contain inosine has been challenge d by the discovery of adenosine deaminases that act on RNA (ADARs). Al though inosine monophosphate (IMP) cannot he detected in crude prepara tions of nucleotides derived from poly(A)(+) RNA, here we show it is r eadily detectable and quantifiable once it is purified away from the W atson-Crick nucleotides. We report that IMP is present in mRNA at tiss ue-specific levels that correlate with the levels of ADAR mRNA express ion. The amount of IMP present in poly(A)(+) RNA isolated from various mammalian tissues suggests adenosine deamination may play an importan t role in regulating gene expression, particularly in brain, where we estimate one IMP is present for every 17 000 ribonucleotides.