Enzymatic trimming

Ribosomal RNAs and transfer RNAs are derived from longer precursor RNAs, trimmed by nucleases. Some bases are modified enzymatically dining the maturation process. 

FIGURE 9.20 Substrate oligosaccharide of enzymatic glycoprotein trimming. 

The cyclohexyl group is commonly found in many medicinal agents, and is also susceptible to mixed-function oxitlti-tion (alicyclic hydroxylation)." Enzymatic introduction of a hydroxyl gmup into a monosubstituted cyclohexane ring generally occurs at C-3 or C-4 and can lead to cis and trims conformational stereoisomers, as shown in the diagrammed. scheme. 

The three principal kinds of RNA—tRNA, rRNA, and niRNA—are all modihed enzymatically after transcription to give rise to the functional form of the RNA in question. The type of processing in prokaryotes can differ greatly from that in eukaryotes, especially in the case of mRNA. The initial size of the RNA transcripts is greater than the hnal size because of leader sequences at the 5 end and trailer sequences at the 3 end. The leader and trailer sequences must be removed, and other forms of trimming are also possible. Terminal sequences can be added after transcription, and base modification is frequently observed, especially in tRNA. 

If the virus was grown in a strain of cell that could not add the first residue of the outer chains, which is N-acetylglucosamine, trimming ceased at GlcNAc2Man5. This implied that addition of A -acetylglucosamine and excision of mannose were linked processes. Such is now known to be the case and Tabas and Kornfeld (1978) have demonstrated the enzymatic basis of this. The glycosylation of G-protein is also modified in glucose-deprived, infected BHK cells (Turco and Pickard, 1982). 

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