The editing of apolipoprotein (apo)B messenger RNA (mRNA) involves a novel C to U modification, which creates an in-frame stop-translation codon, thereby generating the carboxyl-terminal of apoB48. The 27 kDa catalytic subunit of the editing enzyme has been cloned and established to be a zinc-containing cytidine deaminase. The catalytic subunit is guided to the editing site by a second targeting subunit or subunits. A candidate for the targeting subunit is a 60 kDa protein that can be UV crosslinked to the sequence UGAU, which is part of a motif downstream of the editing site that is essential for editing.
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