Inosine

Inosine is a nucleoside consisting of the purine hypoxanthine linked to a ribose sugar by a beta-N9-glycosidic bond. It is the ribonucleoside form of hypoxanthine and a common modified base found in RNA.

In transfer RNA, inosine is notable at the wobble position (position 34). An anticodon containing inosine can pair with codons ending in cytosine, uracil, or adenine, enabling a single tRNA to recognize multiple codons for a given amino acid. At the wobble position in tRNA, inosine is produced by tRNA-specific adenosine deaminases (ADAT).

RNA editing also contributes to inosine formation. Adenosine-to-inosine editing occurs in mRNA and other RNAs via ADAR enzymes, converting adenosine residues to inosine. Inosine is read as guanosine by the translational and splicing machinery, which can alter codons, splice sites, or RNA structure.

Biochemistry and metabolism: Inosine can be phosphorylated to inosine monophosphate (IMP) and then further to diphosphate or triphosphate forms. IMP is an intermediate in purine metabolism and can be salvaged to regenerate nucleotides. Inosine nucleosides are thus connected to both the translational machinery through wobble decoding and to broader post-transcriptional regulation via RNA editing.

Significance: The presence of inosine in tRNA and in RNA editing expands the genetic code's flexibility and provides a mechanism for post-transcriptional regulation of gene expression. In research, inosine-containing RNAs are used to study RNA structure, editing, and codon recognition.