Lysidine (nucleoside)

Lysidine is a modified nucleoside found in the anticodon loop of certain bacterial tRNAs. Specifically, it is a modified form of cytidine (a pyrimidine nucleoside) where the amino group at the 4-position is replaced by a lysyl residue. This modification is crucial for efficient translation of codon AUA, which codes for isoleucine. Without lysidine modification, the tRNA cannot efficiently recognize the AUA codon.

Lysidine is formed post-transcriptionally, meaning it is created after the tRNA molecule has been transcribed from DNA. The enzyme responsible for creating lysidine is typically a lysidine synthetase. The process involves the transfer of the lysyl moiety from lysine to cytidine within the tRNA molecule.

The modification of cytidine to lysidine affects the base-pairing properties of the nucleoside. The presence of the bulky lysyl group alters the shape and charge distribution of the nucleoside, influencing its ability to form stable Watson-Crick base pairs with guanine (G). This altered base-pairing selectivity allows the modified tRNA to specifically recognize the AUA codon while discriminating against other similar codons.

The presence and importance of lysidine are not universal across all organisms. It is predominantly found in bacteria. The absence of lysidine in other organisms highlights the diverse mechanisms employed by different species to achieve accurate and efficient translation. The study of lysidine and its biosynthesis provides valuable insights into the mechanisms of tRNA modification and codon recognition.