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An additional editing site is present in apolipoprotein B mRNA
N Navaratnam1, D Patel, R R Shah
1MRC Clinical Research Centre, Harrow, Middlesex, UK.
Abstract:
Human intestinal apolipoprotein (apo) B mRNA undergoes a C to U RNA editing at nucleotide 6666 to generate a translation stop at codon 2153, which defines the carboxy-terminal of apo B48. Here we show that two of eleven human intestinal cDNAs spanning residue 6666 were edited from a genomically-encoded C to a T at residue 6802 as well as at residue 6666. This additional editing converts Thr (ACA) codon 2198 to Ile (AUA). Synthetic RNA including the nucleotide 6802 was edited in vitro by intestinal extracts at 10-15% of the editing efficiency of nucleotide 6666. A sequence is identified as important for recognition by the editing activity. No secondary structural homology was identified between the two edited sites. No other sequence in the region between 6411 and 6893 nucleotides of apo B mRNA was found to be edited in vivo or in vitro. Apo B RNA editing extracts from intestine did not edit maize cytochrome oxidase II mRNA.
Insights
Human intestinal apolipoprotein B mRNA editing occurs at a second site, nucleotide 6802, in addition to the known site at nucleotide 6666. This discovery reveals a new mechanism in apo B48 protein formation.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- Apolipoprotein B (apo B) mRNA undergoes RNA editing in the human intestine.
- This editing at nucleotide 6666 creates a stop codon, defining apo B48.
Purpose of the Study:
- To investigate additional RNA editing sites in human intestinal apo B mRNA.
- To identify sequences critical for RNA editing recognition.
Main Methods:
- Analysis of human intestinal cDNAs spanning nucleotide 6666.
- In vitro editing assays using synthetic RNA and intestinal extracts.
- Sequence analysis to identify editing recognition sites.
Main Results:
- Two of eleven cDNAs showed editing at both nucleotide 6666 and a novel site, nucleotide 6802.
- Editing at nucleotide 6802 converts a threonine codon to isoleucine.
- In vitro editing efficiency at nucleotide 6802 was 10-15% of that at nucleotide 6666.
- A specific sequence was identified as important for editing recognition.
- No other editing sites were found in the analyzed region, and no structural homology was observed between the two sites.
Conclusions:
- Human intestinal apo B mRNA exhibits editing at a second site (nucleotide 6802) besides the known site (nucleotide 6666).
- This additional editing site contributes to the diversity of apo B protein isoforms.
- The editing process is sequence-specific and does not rely on secondary structural homology between sites.