Apolipoprotein B mRNA editing: a new tier for the control of gene expression.

Two forms of apolipoprotein (apo) B are found in mammals. The shorter form is translated from an edited mRNA in which a specific cytidine base is deaminated to a uridine, creating a new stop codon. Apo B mRNA editing is mediated by a site-specific cytidine deaminase that recognizes a downstream target sequence in the RNA. The enzyme has no energy or cofactor requirements and no RNA component, and thus bears no obvious relationship to RNA processing events such as splicing or polyadenylation. While apo B mRNA editing activity may have arrived late in evolution to target dietary lipid to the liver in mammals, the discovery of the editing activity in tissues and cells that do not express apo B suggests a more widespread role in the generation of RNA and protein diversity.